Provide ${kmodname}-devel-kmod for yum-builddep

In order to ensure that yum-builddep pulls in all the build
requirements a generic ${kmodname}-devel-kmod provides line is
added.  This allows a version of the development headers to be
included without requiring knowledge of the kernel version.

This is important because unlike rpmbuild which does correctly
expand the source rpm spec file, yum-builddep does not.  Without
this generic provides line mock which relies on yum-builddep is
unable to automatically satisfy the dependency.

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