Makefile: remove fedpkg dependency for "srpm"

Makefile: remove fedpkg dependency for "srpm"

It's not currently possible to install EPEL 7's fedpkg alongside the
centos-packager package in CentOS 7 Extras (for /usr/bin/cbs), because
they each depend on slightly different Koji package versions.

This means that we cannot run "make srpm" on a system where
centos-packager is installed, because that depends on fedpkg.

Remove the dependency on fedpkg for the "srpm" target, and run
rpmbuild -bs directly.

The purpose of this change is to make it easier to automatically
build ceph-ansible SRPMs in Jenkins/cbs.centos.org.