runtime(spec): Recognize SourceLicense tag name in RPM spec syntax (#14046)

runtime(spec): Recognize SourceLicense tag name in RPM spec syntax (#14046)

rpm-4.19.0 added a new SourceLicense tag. It is used at the same place
as License tag.

This patch adds the new tag name into a Vim syntax file to be
highligted the same way as the License tag. Note that it has to be
defined in the syntax file before Source\d* regexp. Otherwise it's not
recognized by Vim.

Signed-off-by: Petr Písař <ppisar@redhat.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>