kconfig: add xstrdup() helper

We already have xmalloc(), xcalloc(), and xrealloc(().  Add xstrdup()
as well to save tedious error handling.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/scripts/kconfig/lkc.h b/scripts/kconfig/lkc.h
index 4e23feb..2d5ec2d 100644
--- a/scripts/kconfig/lkc.h
+++ b/scripts/kconfig/lkc.h
@@ -115,6 +115,7 @@ int file_write_dep(const char *name);
 void *xmalloc(size_t size);
 void *xcalloc(size_t nmemb, size_t size);
 void *xrealloc(void *p, size_t size);
+char *xstrdup(const char *s);
 
 struct gstr {
 	size_t len;