mt-st: Add missing header

New version of musl no longer includes this header internally.

Removed several unnecessary variables in Makefile.

Signed-off-by: Rosen Penev <rosenp@gmail.com>
(cherry-picked from 4cda68c72b)

utils/mt-st/patches/010-sysmacros.patch

@@ -0,0 +1,10 @@
+--- a/mt.c
++++ b/mt.c
+@@ -21,6 +21,7 @@
+ #include <sys/types.h>
+ #include <sys/ioctl.h>
+ #include <sys/stat.h>
++#include <sys/sysmacros.h>
+ #include <sys/utsname.h>
+ 
+ #include "mtio.h"