gallium/aux/util/u_debug_describe.c: Silence an -Wunused-param warning

Annotate the unused parameter.

v2: move UNUSED decoration in front of parameter declaration

Signed-off-by: Gert Wollny <gw.fossdev@gmail.com>
Reviewed-by: Brian Paul <brianp@vmware.com> (v1)

src/gallium/auxiliary/util/u_debug_describe.c

@@ -30,7 +30,7 @@
 #include "util/u_string.h"
 
 void
-debug_describe_reference(char* buf, const struct pipe_reference*ptr)
+debug_describe_reference(char* buf, UNUSED const struct pipe_reference*ptr)
 {
    strcpy(buf, "pipe_object");
 }