Add private prototype for g_thread_init_glib()

This function was never put in a header and was only used internally
from libgthread, but we should keep the symbol around to avoid
triggering any false-positives on ABI checkers.

For -Wmissing-prototypes compatibility with this unusual case, we should
add a private prototype in the .c file just before.

glib/deprecated/gthread-deprecated.c

@@ -192,6 +192,7 @@ g_thread_get_initialized (void)
 }
 
 /* We need this for ABI compatibility */
+void g_thread_init_glib (void);
 void g_thread_init_glib (void) { }
 
 /* Internal variables {{{1 */