Add more missing config.h #includes

Now that we use AC_SYS_LARGEFILE, we need to pull in config.h at least
whereever we use mmap().  Fixes at least the test-suite and simple-shm
on 32 bit systems.
diff --git a/tests/weston-test-client-helper.c b/tests/weston-test-client-helper.c
index 264dc0d..0350da5 100644
--- a/tests/weston-test-client-helper.c
+++ b/tests/weston-test-client-helper.c
@@ -20,6 +20,8 @@
  * OF THIS SOFTWARE.
  */
 
+#include <config.h>
+
 #include <stdlib.h>
 #include <stdio.h>
 #include <string.h>