wcn36xx_msm: use KLIB_BUILD instead of KLIB

If not building within the kernel tree, KLIB and KLIB_BUILD can point to
different directories.  Allow for this by conditionally configuring
KLIB, and KLIB_BUILD if they haven't already been setup.
Merge pull request #141 from silverjam/ft-wcn36xx_msm-build-fix

wcn36xx_msm: use KLIB_BUILD instead of KLIB