rpmsg: char: add CONFIG_NET dependency

Without CONFIG_NET, we get a build failure for the new driver:

ERROR: "skb_queue_tail" [drivers/rpmsg/rpmsg_char.ko] undefined!
ERROR: "skb_put" [drivers/rpmsg/rpmsg_char.ko] undefined!
ERROR: "__alloc_skb" [drivers/rpmsg/rpmsg_char.ko] undefined!
ERROR: "kfree_skb" [drivers/rpmsg/rpmsg_char.ko] undefined!
ERROR: "skb_dequeue" [drivers/rpmsg/rpmsg_char.ko] undefined!

This adds a dependency so we don't try to build the broken configuration.

Fixes: c0cdc19f ("rpmsg: Driver for user space endpoint interface")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>