util: Silence GCC unused-but-set-variable warning.

Fix this GCC 4.6 warning with 64-bit builds.
u_debug_stack.c: In function ‘debug_backtrace_capture’:
u_debug_stack.c:45:17: warning: variable ‘frame_pointer’ set but not
used [-Wunused-but-set-variable]

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: José Fonseca <jfonseca@vmware.com>

src/gallium/auxiliary/util/u_debug_stack.c

@@ -86,6 +86,8 @@ debug_backtrace_capture(struct debug_stack_frame *backtrace,
       
       frame_pointer = next_frame_pointer;
    }
+#else
+   (void) frame_pointer;
 #endif
    
    while(nr_frames) {