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 <[e-mail]>
Reviewed-by: Andy Shevchenko <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/uuid.h

···
 #define _LINUX_UUID_H_
 
 #include <uapi/linux/uuid.h>
+#include <linux/string.h>
 
 #define UUID_SIZE 16
 

include/uapi/linux/uuid.h

···
 #define _UAPI_LINUX_UUID_H_
 
 #include <linux/types.h>
-#include <linux/string.h>
 
 typedef struct {
 	__u8 b[16];