fix build of menu-packages (do hardcode /usr)

d2cc121e broke building the menu
packages. The whole point of those is to have hardcoded location, so
that the system's desktop-environment will actually find the
menu/mimetype-definitions and icons

Change-Id: I940eb1243913d21d46d3579de30c8fabdc28fde3

sysui/desktop/freedesktop/freedesktop-menus.spec

@@ -55,6 +55,7 @@ mkdir -p $RPM_BUILD_ROOT
 # set parameters for the create_tree script
 export DESTDIR=$RPM_BUILD_ROOT
 export KDEMAINDIR=/usr
+export PREFIXDIR=/usr
 export GNOMEDIR=%{gnome_dir}
 
 ./create_tree.sh