Add missing headers fort ports-gcc

Index: src/scim_hangul_imengine.cpp
--- src/scim_hangul_imengine.cpp.orig
+++ src/scim_hangul_imengine.cpp
@@ -48,6 +48,9 @@
   #define bind_textdomain_codeset(domain,codeset)
 #endif
 
+#include <string.h>
+#include <unistd.h>
+
 #define scim_module_init hangul_LTX_scim_module_init
 #define scim_module_exit hangul_LTX_scim_module_exit
 #define scim_imengine_module_init hangul_LTX_scim_imengine_module_init