usb: gadget: Allow f_rndis and f_gsi to coexist as modules

Currently the make targets for CONFIG_USB_F_RNDIS and
CONFIG_USB_F_GSI both include rndis.o. This would work if only
one is enabled, or both are enabled as =Y as rndis.o will get
linked into the vmlinux image once. Otherwise if both are
enabled with at least one as a module, it results in a module
load failure due to duplicate symbols.

A simple fix for this is to let the usb_f_gsi.o module only
include rndis.o if CONFIG_USB_F_RNDIS is disabled.

Change-Id: Id5b96aaa9a728aa26a65b2c66f2341d6e2fb786b
Signed-off-by: Jack Pham <jackp@codeaurora.org>