Include config.h in files that test for constants defined in it.

diff --git a/memory/selector.c b/memory/selector.c
index 331bdef..9d31b2b 100644
--- a/memory/selector.c
+++ b/memory/selector.c
@@ -5,6 +5,8 @@
  */
 
 #include <string.h>
+
+#include "config.h"
 #include "winerror.h"
 #include "wine/winbase16.h"
 #include "ldt.h"