phosphor-certificate-manager: simplify FILES directive

The recipe has collected a bunch of needless FILES settings.  Simplify
to the current minimum required set.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I0da538e22ea5d1284f32bd2d2c22bcb61db39bd1
diff --git a/meta-phosphor/recipes-phosphor/certificate/phosphor-certificate-manager_git.bb b/meta-phosphor/recipes-phosphor/certificate/phosphor-certificate-manager_git.bb
index 371967d..9f2c745 100644
--- a/meta-phosphor/recipes-phosphor/certificate/phosphor-certificate-manager_git.bb
+++ b/meta-phosphor/recipes-phosphor/certificate/phosphor-certificate-manager_git.bb
@@ -43,4 +43,4 @@
         ${@bb.utils.contains('PACKAGECONFIG', 'bmcweb', 'phosphor-certificate-manager@bmcweb.service', '', d)} \
         "
 
-FILES:${PN}:append = " ${sysconfdir}/dbus-1 ${base_libdir} ${datadir} ${bindir}"
+FILES:${PN}:append = " ${systemd_system_unitdir}/*"