build: require yajl >= 2.0.3

build: require yajl >= 2.0.3

The pkg-config file was introduced by commit b729ded which was released
in yajl 2.0.3.

Since all our supported platforms include at least yajl 2.0.4,
use pkg-config to detect the library and set the minimum to 2.0.3.

https://repology.org/project/yajl/versions

Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>