vc4: make vc4_begin_query() return a boolean

I forgot to make the change in 96f164f6f0.
This fixes a warning with GCC and probably an error with Clang.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>

src/gallium/drivers/vc4/vc4_query.c

         free(query);
 }
 
-static void
+static boolean
 vc4_begin_query(struct pipe_context *ctx, struct pipe_query *query)
 {
+        return true;
 }
 
 static void