LU-12511 build: don't use OpenSFS UAPI headers with --disable-modules

LU-12511 build: don't use OpenSFS UAPI headers with --disable-modules

If we configure Lustre with --disable-modules that means we want
to use the native Linux Lustre / LNet UAPI headers instead. To do
this only include OpenSFS UAPI header path when Lustre modules are
enabled.

Change-Id: Id0f4d960d51bf02a6e24d2e3ad2544df10384533
Test-Parameters: trivial
Signed-off-by: James Simmons <jsimmons@infradead.org>
Reviewed-on: https://review.whamcloud.com/38370
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Nathaniel Clark <nclark@whamcloud.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>