screen: fix musl compatibility

Add missing `os.h` include to `utmp.c` to pull in the required `utmp.h` header.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

utils/screen/patches/200-musl-compat.patch

@@ -0,0 +1,10 @@
+--- a/utmp.c
++++ b/utmp.c
+@@ -33,6 +33,7 @@
+ #include "config.h"
+ #include "screen.h"
+ #include "extern.h"
++#include "os.h"
+ 
+ #ifdef HAVE_UTEMPTER
+ #include <utempter.h>