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>

include/linux/string.h

@@ -144,8 +144,8 @@ static inline bool strstarts(const char *str, const char *prefix)
 {
 	return strncmp(str, prefix, strlen(prefix)) == 0;
 }
-#endif
 
 extern size_t memweight(const void *ptr, size_t bytes);
 
+#endif /* __KERNEL__ */
 #endif /* _LINUX_STRING_H_ */