mesa: return INVALID_VALUE from WaitSync if flags != 0

Fixes gles3conform's sync_error_waitsync_flags test.
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/mesa/main/syncobj.c

@@ -352,7 +352,7 @@ _mesa_WaitSync(GLsync sync, GLbitfield flags, GLuint64 timeout)
    }
 
    if (flags != 0) {
-      _mesa_error(ctx, GL_INVALID_ENUM, "glWaitSync(flags=0x%x)", flags);
+      _mesa_error(ctx, GL_INVALID_VALUE, "glWaitSync(flags=0x%x)", flags);
       return;
    }