kconfig: add static qualifier to expand_string()

Now expand_string() is only used in preprocess.c

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

scripts/kconfig/lkc_proto.h

@@ -58,7 +58,6 @@ void env_write_dep(FILE *f, const char *auto_conf_name);
 void variable_add(const char *name, const char *value,
 		  enum variable_flavor flavor);
 void variable_all_del(void);
-char *expand_string(const char *in);
 char *expand_dollar(const char **str);
 char *expand_one_token(const char **str);