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
diff --git a/meta-openembedded/meta-oe/recipes-support/libcereal/libcereal_1.2.2.bb b/meta-openembedded/meta-oe/recipes-support/libcereal/libcereal_1.2.2.bb
index e02b4c9..365b40b 100644
--- a/meta-openembedded/meta-oe/recipes-support/libcereal/libcereal_1.2.2.bb
+++ b/meta-openembedded/meta-oe/recipes-support/libcereal/libcereal_1.2.2.bb
@@ -15,4 +15,6 @@
 
 EXTRA_OECMAKE = "-DJUST_INSTALL_CEREAL=ON"
 
+ALLOW_EMPTY_${PN} = "1"
+
 BBCLASSEXTEND = "native nativesdk"