kbuild : Fix documentation of INSTALL_HDR_PATH

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: Anish Bhatt <anish@chelsio.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>