gitk: fix tabbed preferences construction when using tcl 8.4

In 8.5 the incr command creates the target variable if it does not exist
but in 8.4 using incr on a non-existing variable raises an error. Ensure
we have created our counter variable when creating the tabbed dialog for
non-themed preferences.
Reported-by: NRamsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: NPat Thoyts <patthoyts@users.sourceforge.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

gitk

@@ -11012,6 +11012,7 @@ proc doprefs {} {
     lappend pages [prefspage_general $notebook] [mc "General"]
     lappend pages [prefspage_colors $notebook] [mc "Colors"]
     lappend pages [prefspage_fonts $notebook] [mc "Fonts"]
+    set col 0
     foreach {page title} $pages {
 	if {$use_notebook} {
 	    $notebook add $page -text $title