uuid: cleanup <uapi/linux/uuid.h>

Exported header doesn't use anything from <linux/string.h>,
it is <linux/uuid.h> which uses memcmp().

Link: http://lkml.kernel.org/r/20171225171121.GA22754@avx2
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@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/uuid.h b/include/linux/uuid.h
index 33b0bdb..d9c4a6c 100644
--- a/include/linux/uuid.h
+++ b/include/linux/uuid.h
@@ -17,6 +17,7 @@
 #define _LINUX_UUID_H_
 
 #include <uapi/linux/uuid.h>
+#include <linux/string.h>
 
 #define UUID_SIZE 16