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: c0cdc19f84a4 ("rpmsg: Driver for user space endpoint interface")
Signed-off-by: Arnd Bergmann <[e-mail]>
Signed-off-by: Bjorn Andersson <[e-mail]>

drivers/rpmsg/Kconfig

···
 config RPMSG_CHAR
 	tristate "RPMSG device interface"
 	depends on RPMSG
+	depends on NET
 	help
 	  Say Y here to export rpmsg endpoints as device files, usually found
 	  in /dev. They make it possible for user-space programs to send and