lib: Make sure -lselinux is used when linking libguestfs.so.

If libselinux was detected, it was not added to the linker command
line.  This still worked (at least on Fedora) because some other
library pulls in the dependency implicitly, possibly libvirt.  However
this broke on Arch:

https://aur.archlinux.org/packages/libguestfs/

Reported by and thanks: Antoni Segura Puimedon.