[PATCH] uml: add a comment explaining pread availability

Prompted by Dave Mielke: This adds a comment explaining why we do not
define _XOPEN_SOURCE, even though the pread man page says you should.
Signed-off-by: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

arch/um/drivers/cow_user.c

 #include <stddef.h>
 #include <string.h>
 #include <errno.h>
+/* _XOPEN_SOURCE is needed for pread, but we define _GNU_SOURCE, which defines
+ * that.
+ */
 #include <unistd.h>
 #include <byteswap.h>
 #include <sys/time.h>