build: Add missing "static" keyword where it should be used

Otherwise we fail to build with -Werror=missing-prototypes.

https://bugzilla.gnome.org/show_bug.cgi?id=687385

tests/testgdate.c

@@ -29,7 +29,8 @@ else \
   if ((passed+notpassed) % 10000 == 0) g_print ("."); fflush (stdout); \
 } G_STMT_END
 
-void g_date_debug_print(GDate* d)
+static void
+g_date_debug_print (GDate* d)
 {
   if (!d) g_print("NULL!\n");
   else