Fix KLIB_BUILD definition

you should not use in any of the kmod spec the:
/lib/modules/%{kversion}/build
since it's part of the kernel package (not the kernel-devel) which is
never installed in a mock environment.
this simple patch fix this problem.

Signed-off-by: Farkas Levente <lfarkas@lfarkas.org>
Signed-off-by: Philip J Perry <phil@elrepo.org>