[SPM] Workaround for header only GTMDefines SPM module

[dshamaev-intermedia]

• added dummy source file for GTMDefines SPM module to mitigate Header-only targets support swiftlang/swift-package-manager#5706
• removed extra commas from Package.swift which were causing compatibility issues with Swift toolset bundled within Xcode 16.2

[google-cla]

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[thomasvl]

@dshamaev-intermedia please seen the cla bot reply, you'll need to do that before we can look at the PR.

[jj3028767-max]

Quisiera aser cambio