Add missing include in kicad_string.h

Without this include the HAVE_STRTOKR define won't be available to
the preprocessor and the strtok_r function will be declared
unconditionally and eventually clash with the declaration from the
system provided equivalent.

include/kicad_string.h

@@ -30,6 +30,8 @@
 #ifndef KICAD_STRING_H_
 #define KICAD_STRING_H_
 
+#include "config.h"
+
 #include <wx/string.h>
 #include <wx/filename.h>