vrend: Add #define for unused variables

This is required to silence -Wunused-parameter warnings.

v2: - Use mesa-style UNUSED for unused parameters
    - Add it it gallium/aux/os_misc.h instead of util/u_debug.h, here it seems
      a bit more apropriate

Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Signed-off-by: Jakob Bornecrantz <jakob@collabora.com>

src/gallium/auxiliary/os/os_misc.h

@@ -43,6 +43,13 @@
 #  include <unistd.h> /* getpid() */
 #endif
 
+#ifdef __GNUC__
+#define UNUSED __attribute__((unused))
+#define MAYBE_UNUSED __attribute__((unused))
+#else
+#define UNUSED
+#define MAYBE_UNUSED
+#endif
 
 #ifdef  __cplusplus
 extern "C" {