main/skalibs: specify libdir explicitly
Otherwise, libskalibs.a is installed to the libexecdir (i.e. /usr/lib/skalibs/libskalibs.a). Since this directory is not added to the linker path in the pkg-config file, this causes software which statically links against skalibs (e.g. BusyBox) to no longer link successfully.
Regression introduced in 8e846805.
Alternative solution: Modify the pkg-config file accordingly.