Add missing header for ports-gcc

Index: src/scim_pinyin.cpp
--- src/scim_pinyin.cpp.orig
+++ src/scim_pinyin.cpp
@@ -28,6 +28,8 @@
 #define Uses_SCIM_CONFIG_PATH
 #define Uses_SCIM_LOOKUP_TABLE
 
+#include <string.h>
+
 #include <scim.h>
 #include "scim_pinyin.h"
 
