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.

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

meta-oe/recipes-support/libcereal/libcereal_1.2.2.bb

@@ -15,4 +15,6 @@ inherit cmake
 
 EXTRA_OECMAKE = "-DJUST_INSTALL_CEREAL=ON"
 
+ALLOW_EMPTY_${PN} = "1"
+
 BBCLASSEXTEND = "native nativesdk"