headers: avoid bareword attributes

Always use the __-decorated form of an attribute name in public
headers, as the bareword form is in the user's namespace, and we
don't want compilation to break just because the user defines the
bareword to mean something else.

Signed-off-by: Eric Blake <eblake@redhat.com>

winsup/cygwin/include/cygwin/config.h

@@ -41,7 +41,7 @@ extern "C" {
 #else
 #include "../tlsoffsets.h"
 #endif
-__attribute__((gnu_inline))
+__attribute__((__gnu_inline__))
 extern inline struct _reent *__getreent (void)
 {
   register char *ret;