Fix zfs.spec.in to use @LICENSE@ variable

The zfs.spec.in file had the license field hard coded to specify the
CDDL. This was changed to use the @LICENSE@ variable, maintaining
consistency with the zfs-modules.spec.in file.

Signed-off-by: Prakash Surya <surya1@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

zfs.spec.in

@@ -12,7 +12,7 @@ Group:           Utilities/System
 Name:            %{name}
 Version:         %{version}
 Release:         %{release}
-License:         CDDL
+License:         @LICENSE@
 URL:             git://github.com/zfsonlinux/zfs.git
 BuildRoot:       %{_tmppath}/%{name}-%{version}-%{release}-%(%{__id} -un)
 Source:          %{name}-%{version}.tar.gz