glib-mkenums.in: fix @ENUMPREFIX@ with /*< underscore_name=... >*/

For enums defined with an underscore_name override, @ENUMPREFIX@
would include a trailing "_". Fix.

https://bugzilla.gnome.org/show_bug.cgi?id=661797

gobject/glib-mkenums.in

@@ -379,7 +379,7 @@ while (<>) {
 	    $enumshort =~ s/^[A-Z][A-Z0-9]*_//;
 
 	    $enumname_prefix = $enumlong;
-	    $enumname_prefix =~ s/$enumshort$//;
+	    $enumname_prefix =~ s/_$enumshort$//;
 	} else {
 	    # enumname is e.g. GMatchType
 	    $enspace = $enumname;