Stop masking the root file system mount unit (!116)

Masking the root file system (-.mount) unit lead to a Debian package
upgrade failing as reported here [1].  This was fixed in systemd 245
[2][3] by not allowing perpetual units to be masked.  As the root file
system can't be mounted or unmounted while GParted is running, it
doesn't need to be prevented by masking the unit.  Therefore stop
masking the root file system mount unit.

[1] Debian bug #948710 -  handle masked .mount unit more gracefully
    https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=948710
[2] systemd issue #14550 - Handle masked .mount units more gracefully
    https://github.com/systemd/systemd/issues/14550
[3] core: never allow perpetual units to be masked
    88414eed6f

Closes !116 - Systemd mount masking and udev rule location updates

gparted.in

@@ -158,6 +158,7 @@ if test "x$HAVE_SYSTEMCTL" = "xyes"; then
 		if (substr(what,1,5) == "/dev/"     &&
 		    loadstate        != "masked"    &&
 		    what             != "/dev/fuse" &&
+		    where            != "/"         &&
 		    ! (substr(what,1,9) == "/dev/loop" && substr(where,1,6) == "/snap/"))
 		{
 			print id