PREFIXDIR must be /usr for .deb system integration package

... tools don't find *.desktop files in /usr/local or elsewhere,
and the applications will not be registered in menus. Fixed the
regression from d2cc121e

Change-Id: I679b98b5cd0a502553fb13025ee8b9ac4bb5507e

sysui/CustomTarget_deb.mk

@@ -46,6 +46,7 @@ $(deb_WORKDIR)/%/DEBIAN/control: $(deb_SRCDIR)/control $(call gb_CustomTarget_ge
 		ICON_PREFIX=$(UNIXFILENAME.$*) \
 		KDEMAINDIR=/usr \
 		GNOMEDIR=/usr \
+		PREFIXDIR=/usr \
 		./create_tree.sh
 	sed $(deb_SRCDIR)/openoffice.org-debian-menus \
 		-e 's/%PRODUCTNAME/$(PRODUCTNAME.$*) $(PRODUCTVERSION)/' \