Make sure size_t is defined before used in strings.h

Otherwise, including strings.h as the first file in a C file yields errors.

This fixes issue 3332.
diff --git a/libc/include/strings.h b/libc/include/strings.h
index 1f73e21..fee7dc4 100644
--- a/libc/include/strings.h
+++ b/libc/include/strings.h
@@ -39,6 +39,7 @@
 #ifndef _STRINGS_H_
 #define _STRINGS_H_
 
+#include <sys/types.h>
 #include <sys/cdefs.h>
 
 __BEGIN_DECLS