fix darshan-apmpi.c

[wkliao]

This PR fixes file darshan-apmpi.c by making variable __darshan_disabled
local to its caller functions. Otherwise, building Darshan 3.4.7 with configure
command-line option --enable-apmpi-mod --enable-apmpi-coll-sync would
fail with the error messages below.

In file included from darshan-apmpi.c:18:
darshan-apmpi.c: In function ‘__wrap_MPI_Barrier’:
/home/runner/work/darshan/darshan/darshan-runtime/lib/darshan.h:227:9:
error: redefinition of ‘__darshan_disabled’
227 | int __darshan_disabled =
darshan_core_disabled_instrumentation()
| ^~~~~~~~~~~~~~~~~~
darshan-apmpi.c:32:11: note: in expansion of macro ‘MAP_OR_FAIL’
32 | MAP_OR_FAIL(PMPI_Barrier);
| ^~~~~~~~~~~
darshan-apmpi.c:1389:5: note: in expansion of macro ‘TIME_SYNC’
1389 | TIME_SYNC(__real_PMPI_Barrier(comm));
| ^~~~~~~~~

Darshan 3.5.0, to be released soon, has incorporated this same fix in
darshan-hpc/darshan#1082

cc @carns @roblatham00