core: remove ending parenthesis in ifdef condition

warning: ISO C99 requires whitespace after the macro name
sched_yield.h:34:20: warning: extra tokens at end of #ifndef directive
 #ifndef sched_yield()

- reported by Victore Seva, GH #576

sched_yield.h

@@ -31,7 +31,7 @@
 #else
 #include <unistd.h>
 	/* fake sched_yield */
-#ifndef sched_yield()
+#ifndef sched_yield
 	#define sched_yield()	sleep(0)
 #endif
 #endif