Update dispatch/base.h

Co-authored-by: Evan Wilde <etceterawilde@gmail.com>

Author: compnerd

dispatch/base.h

@@ -187,7 +187,7 @@
 #endif
 
 #if defined(dispatch_STATIC)
-# if defined(__GNUC__)
+# if __GNUC__
 #   define DISPATCH_EXPORT DISPATCH_EXTERN __attribute__((__visibility__("hidden")))
 # else
 #   define DISPATCH_EXPORT DISPATCH_EXTERN