Use BUILD_DEPENDS option for dkms builds

Support was added to dkms so build dependencies can be specified.
This allows us to ensure that the spl package will always be built
before the zfs package.  Those patches have not yet been merged
upstream but they are available in the zfsonlinux/dkms repository.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

scripts/dkms.mkconf

@@ -52,6 +52,7 @@ POST_INSTALL="scripts/dkms.postinst
   -k \${kernelver}
   -t \${dkms_tree}
 "
+BUILD_DEPENDS[0]="spl"
 AUTOINSTALL="yes"
 REMAKE_INITRD="no"
 MAKE[0]="make"