exports.c: fix warning: 'dummy' defined but not used

Also get rid of the #ifdef's while doing this.

Suggested-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Wolfgang Denk <wd@denx.de>

common/exports.c

@@ -3,7 +3,7 @@
 
 DECLARE_GLOBAL_DATA_PTR;
 
-static void dummy(void)
+__attribute__((unused)) static void dummy(void)
 {
 }