vc4: Just put USE_VC4_SIMULATOR in DEFINES.

In the pipe-loader reworks, it was missed in one of the new directories it
was used.

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
diff --git a/configure.ac b/configure.ac
index 91fdfe5..120c93e 100644
--- a/configure.ac
+++ b/configure.ac
@@ -2173,7 +2173,9 @@
             gallium_require_drm_loader
 
             PKG_CHECK_MODULES([SIMPENROSE], [simpenrose],
-                              [USE_VC4_SIMULATOR=yes], [USE_VC4_SIMULATOR=no])
+                              [USE_VC4_SIMULATOR=yes;
+                               DEFINES="$DEFINES -DUSE_VC4_SIMULATOR"],
+                              [USE_VC4_SIMULATOR=no])
             ;;
         xvirgl)
             HAVE_GALLIUM_VIRGL=yes