string: Include the config header

At present the config header is not included in this file, but it does use
a CONFIG option. Fix it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/lib/string.c b/lib/string.c
index c4ca944..af17c16 100644
--- a/lib/string.c
+++ b/lib/string.c
@@ -15,6 +15,7 @@
  *    reentrant and should be faster). Use only strsep() in new code, please.
  */
 
+#include <config.h>
 #include <linux/types.h>
 #include <linux/string.h>
 #include <linux/ctype.h>