ZFS-kmod.spec: use $SPL passed by --with-spl ..

- use $SPL as @SPL@ passed by configure --with-spl=/path/to/spl
- verified to work on Debian Wheezy (7.x)
===================
I couldn't figure why "make deb-kmod" was failing, even though I've explicitly ran "./configure --with-spl=/home/anapsix/spl-0.6.1", until I've found spec.in ignored the flag
Why not use $SPL passed from ./configure for spec file generation?