dbus: fix cmake variable nameing for QTHELP

The cmake variable ENABLE_QT_HELP does no exist. Use instead the
variable DBUS_ENABLE_QTHELP_DOCS.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>

utils/dbus/Makefile

@@ -88,7 +88,7 @@ CMAKE_OPTIONS += \
 	-DDBUS_DISABLE_CHECKS=ON \
 	-DDBUS_BUILD_X11=OFF \
 	-DDBUS_ENABLE_DOXYGEN_DOCS=OFF \
-	-DENABLE_QT_HELP=OFF
+	-DDBUS_ENABLE_QTHELP_DOCS=OFF
 
 define Build/InstallDev
 	$(INSTALL_DIR) $(1)/usr/include