build: Guard the inclusions of config.h with HAVE_CONFIG_H

autoconf can be configured to not generate a config.h but to give the
defines with command line arguments instead. In this case, there's no
config.h to include.

To work in both cases autoconf adds a HAVE_CONFIG_H define on the command
line to signal there's a config.h to include.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
diff --git a/tests/gem_fence_thrash.c b/tests/gem_fence_thrash.c
index 3d50e33..c9e847b 100644
--- a/tests/gem_fence_thrash.c
+++ b/tests/gem_fence_thrash.c
@@ -26,7 +26,9 @@
  *
  */
 
+#ifdef HAVE_CONFIG_H
 #include "config.h"
+#endif
 
 #include <unistd.h>
 #include <stdlib.h>