libcereal: Allow empty package

libcereal is header-only so create the empty base package so that
libcereal can be sanely included in an SDK via IMAGE_INSTALL, RDEPENDS,
and the like.

(cherry picked from meta-openembedded commit e23284f9a57537f7dcaca4cd068f3862e205188a)

Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
Signed-off-by: Khem Raj <raj.khem@gmail.com>
Change-Id: I6356cf78c2bc461cab32a1fd443a9163129c3f86