net/rpmsg: remove rwlock.h header inclusion

The rpmsg-proto driver is currently including the linux/rwlock.h
header file, and it is not recommended to be including this header
file directly. So remove the inclusion of this header file.

This also fixes a build error when building on a RT-enabled kernel.

Reported-by: Dan Murphy <dmurphy@ti.com>
Signed-off-by: Suman Anna <s-anna@ti.com>