rhel: Add optional BuildRequires to libcap-ng

rhel: Add optional BuildRequires to libcap-ng

Commit e91b927d8 (lib/daemon: support --user option for all OVS daemon)
added optional usage of the libcap-ng library.  It's packaged in Fedora,
so go ahead and added it by default to the Fedora spec file.

Our default systemd unit files don't make use of the --user option that
requires this library, but conceivably someone may want to customize
them and use this option.

For those that don't want to use --user option, the Fedora package
offers an option (--without libcapng) to build the RPMs without it.

Signed-off-by: Flavio Leitner <fbl@redhat.com>
Acked-by: Russell Bryant <rbryant@redhat.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>