Fix using wrong SYSDIR after r331683.

This was inadvertently overriding the first found SYSDIR with the last
of /usr/src which could result in the wrong headers being used if not
building from /usr/src.

SYSDIR?= is not used here to avoid evaluating the exists() when unneeded.

Reported by: rgrimes, sjg, Mark Millard
Pointyhat to: bdrewery
Sponsored by: Dell EMC