um: Include sys/types.h for makedev(), major(), minor()

The functions in question are not part of the POSIX standard,
documentation however hints that the corresponding header shall
be sys/types.h. C libraries other than glibc, namely musl, did
not include that header via other ways and complained.

Signed-off-by: Hans-Werner Hilse <hwhilse@gmail.com>
Signed-off-by: Richard Weinberger <richard@nod.at>
diff --git a/arch/um/os-Linux/file.c b/arch/um/os-Linux/file.c
index 08d90fb..26e0164 100644
--- a/arch/um/os-Linux/file.c
+++ b/arch/um/os-Linux/file.c
@@ -13,6 +13,7 @@
 #include <sys/socket.h>
 #include <sys/stat.h>
 #include <sys/un.h>
+#include <sys/types.h>
 #include <os.h>
 
 static void copy_stat(struct uml_stat *dst, const struct stat64 *src)