gmain: Make GSourceCallback thread-safe

Otherwise there is a race in finalising the GSourceCallback if one
thread is finishing off a g_main_dispatch() while another thread is
destroying the GSource which owns the GSourceCallback.

A helgrind log:

==21707== Possible data race during write of size 4 at 0x54EACB0 by
thread #12
==21707== Locks held: none
==21707==    at 0x4ECC174: g_source_callback_unref (gmain.c:1528)
==21707==    by 0x4ECD953: g_main_dispatch (gmain.c:3081)
==21707==    by 0x4ECE667: g_main_context_dispatch (gmain.c:3673)
==21707==    by 0x4ECE859: g_main_context_iterate (gmain.c:3744)
==21707==    by 0x4ECEC7F: g_main_loop_run (gmain.c:3938)
==21707==    by 0x41C197: some_thread (some-code.c:224)
==21707==
==21707== This conflicts with a previous write of size 4 by thread #5
==21707== Locks held: 1, at address 0x54CF320
==21707==    at 0x4ECC174: g_source_callback_unref (gmain.c:1528)
==21707==    by 0x4ECB86F: g_source_destroy_internal (gmain.c:1178)
==21707==    by 0x4ECB9D4: g_source_destroy (gmain.c:1227)
==21707==    by 0x41CF09: some_other_thread (some-other-code.c:410)

https://bugzilla.gnome.org/show_bug.cgi?id=737677