gdm: fix build with gcc15

[ghpzin]

• add patch from merged upstream MR:
  https://gitlab.gnome.org/GNOME/gdm/-/merge_requests/273

Fixes build failure with gcc15:

FAILED: [code=1] common/libgdmcommon.a.p/gdm-settings-utils.c.o
In file included from ../common/gdm-settings-utils.c:38:
../common/gdm-settings-utils.h:47:77: error: expected ';', ',' or ')' before 'bool'
   47 |                                                                 gboolean   *bool);
      |                                                                             ^~~~
../common/gdm-settings-utils.c:290:50: error: expected ';', ',' or ')' before 'bool'
  290 |                                      gboolean   *bool)
      |                                                  ^~~~
...
FAILED: [code=1] common/libgdmcommon.a.p/gdm-settings-direct.c.o
In file included from ../common/gdm-settings-direct.c:38:
../common/gdm-settings-utils.h:47:77: error: expected ';', ',' or ')' before 'bool'
   47 |                                                                 gboolean   *bool);
      |                                                                             ^~~~
../common/gdm-settings-direct.c: In function 'gdm_settings_direct_get_boolean':
../common/gdm-settings-direct.c:159:15: error: implicit declaration of function
'gdm_settings_parse_value_as_boolean'; did you mean
'gdm_settings_parse_value_as_double'? [-Wimplicit-function-declaration]
  159 |         ret = gdm_settings_parse_value_as_boolean  (str, value);
      |               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |               gdm_settings_parse_value_as_double

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Tested build with:

nix-build --expr 'with import ./. {}; gdm.override { stdenv = gcc15Stdenv; }'

Part of fixes for gcc15 update:
#440456

Things done

• Built on platform:
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• Tested, as applicable:
  • NixOS tests in nixos/tests.
  • Package tests at passthru.tests.
  • Tests in lib/tests or pkgs/test for functions and "core" functionality.
• Ran nixpkgs-review on this PR. See nixpkgs-review usage.
• Tested basic functionality of all binary files, usually in ./result/bin/.
• Nixpkgs Release Notes
  • Package update: when the change is major or breaking.
• NixOS Release Notes
  • Module addition: when adding a new NixOS module.
  • Module update: when the change is significant.
• Fits CONTRIBUTING.md, pkgs/README.md, maintainers/README.md and other READMEs.

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[ghpzin]

Merged commit is in 49.rc tag.
gdm should be updated to it in:
#440720
4c1a2f5
Will keep it in draft for now, in case something changes.

[ghpzin]

#440720 was merged with gdm update