The header install makefile creates an 'include' directory inside
INSTALL_HDR_PATH and appending include to the path results in headers
being installed to include/include.

Don't recommend appending include to the path as makefile already does
this.
Signed-off-by: NAnish Bhatt <anish@chelsio.com>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>