meta-phosphor: enable 'usrmerge'

An upcoming systemd is going to deprecate split-usr support[1] and
Yocto supports 'usrmerge' as a DISTRO_FEATURE.  Getting out in front
of the upcoming systemd release by enabling this in meta-phosphor now.

[1]: https://lists.freedesktop.org/archives/systemd-devel/2022-September/048352.html

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I6d95ee3fccf35f38b4104da955eae4c19d76bd50
diff --git a/meta-phosphor/conf/distro/include/phosphor-base.inc b/meta-phosphor/conf/distro/include/phosphor-base.inc
index b1f8d59..d04cafa 100644
--- a/meta-phosphor/conf/distro/include/phosphor-base.inc
+++ b/meta-phosphor/conf/distro/include/phosphor-base.inc
@@ -107,6 +107,7 @@
         slp \
         systemd \
         seccomp \
+        usrmerge \
         "
 
 DISTRO_FEATURES += " \