um: Compile with modern headers

Recent libcs have gotten a bit more strict, so we actually need to
include the right headers and use the right types. This enables UML to
compile again.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Cc: stable@vger.kernel.org
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/os-Linux/file.c

@@ -12,6 +12,7 @@
 #include <sys/mount.h>
 #include <sys/socket.h>
 #include <sys/stat.h>
+#include <sys/sysmacros.h>
 #include <sys/un.h>
 #include <sys/types.h>
 #include <os.h>