Update kernel rpmsg header path in message handler

The source files for the Linux kernel driver has been restructured and
the common rpmsg header file has been renamed and moved to an include
folder.

The message handler openamp application has been updated accordingly to
use the new header file.

Change-Id: If1c4f5f80dbd87ee1b14103620b1183f6a3e3826
Signed-off-by: Mikael Olsson <mikael.olsson@arm.com>