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 d2cc121e0798a007a6e0409b09c16ce747ab959f

Change-Id: I679b98b5cd0a502553fb13025ee8b9ac4bb5507e
diff --git a/sysui/CustomTarget_deb.mk b/sysui/CustomTarget_deb.mk
index ea68e2f..8a48894 100644
--- a/sysui/CustomTarget_deb.mk
+++ b/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)/' \