eal/bsd: fix kernel modules build with meson

eal/bsd: fix kernel modules build with meson

The kernel module source file directory passed via VPATH was wrong,
which caused the source files to be not found via make. Rather than
explicitly passing VPATH, make use of the fact that the full path
to the source files is passed by meson, so split that into directory
part - to be used as VPATH - and file part - to be used as the source
filename.

Fixes: 610beca42ea4 ("build: remove library special cases")

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>