string: do not export memweight() to userspace

Fix the following warning:

  usr/include/linux/string.h:8: userspace cannot reference function or variable defined in the kernel

Signed-off-by: WANG Cong <xiyou.wangcong@gmail.com>
Acked-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/include/linux/string.h b/include/linux/string.h
index ffe0442..b917881 100644
--- a/include/linux/string.h
+++ b/include/linux/string.h
@@ -144,8 +144,8 @@
 {
 	return strncmp(str, prefix, strlen(prefix)) == 0;
 }
-#endif
 
 extern size_t memweight(const void *ptr, size_t bytes);
 
+#endif /* __KERNEL__ */
 #endif /* _LINUX_STRING_H_ */