phosphor-logging: fix yaml option

The meson option in the repository is 'yamldir' and not 'yaml_dir'.
Fix this since later versions of Meson complain about an unknown
option being passed.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I4241745e6cafe310152e8d8235db012af447a167
1 file changed