Fix syntax error in progressbar.sh

Fixes issue 3624

Change-Id: I7fbc5c61131af95998a1617ad1c83a231c556526
Reviewed-on: http://gerrit.dmdirc.com/665
Reviewed-by: Shane Mc Cormack <shane@dmdirc.com>
Automatic-Compile: Gregory Holmes <greg@dmdirc.com>
Reviewed-by: Gregory Holmes <greg@dmdirc.com>

installer/linux/progressbar.sh

 	readprogress
 	CONTINUE="0"
 	echo "Progress Bar Complete"
-elif [ "" ! "${ZENITY}" -a "" != "${DISPLAY}" ]; then
+elif [ "" != "${ZENITY}" -a "" != "${DISPLAY}" ]; then
 	# Else, if zenity exists and we have a display
 	echo "Progress dialog on Display: ${DISPLAY}"
 	TYPE="GNOME"
 	echo "Finished!"
 fi
 echo "Exiting progressbar"
-exit 0;
+exit 0;