v3d: fix empty-body instruction

Fixes: 8d43e2b2de ("meson: add -Werror=empty-body to disallow `if(x);`")
Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/broadcom/cle/v3d_packet_helpers.h

@@ -39,7 +39,7 @@
 #define __gen_validate_value(x) VALGRIND_CHECK_MEM_IS_DEFINED(&(x), sizeof(x))
 #endif
 #else
-#define VG(x)
+#define VG(x) ((void)0)
 #endif
 
 #ifndef __gen_validate_value