meson: improve subproject builds

Add meson subproject wrapfiles for all the openbmc projects we utilize
and fix the missing dependency for cereal.  There is still a missing
dependency for the `ldap.h` that prevents a full wrap-based build.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: If3a5836c3ff9a34d5624c60ac00c8fd46bfba20b
diff --git a/phosphor-ldap-mapper/meson.build b/phosphor-ldap-mapper/meson.build
index d2f4c50..a3a027e 100644
--- a/phosphor-ldap-mapper/meson.build
+++ b/phosphor-ldap-mapper/meson.build
@@ -1,4 +1,5 @@
 phosphor_ldap_mapper_deps = [
+    cereal_dep,
     phosphor_dbus_interfaces_dep,
     phosphor_logging_dep,
     sdbusplus_dep,