meson: switch code dependency to libsystemd

For a while now systemd has packaged a separate `libsystemd`.  For
meson dependency, `systemd` should be used when looking for systemd
service file paths and `libsystemd` should be used for `sd_*` functions.

Update the dependency accordingly.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I8f0c1e052190bdda2b626e43dbb5b807b8b5257f
diff --git a/meson.build b/meson.build
index bea1b76..9030537 100644
--- a/meson.build
+++ b/meson.build
@@ -38,7 +38,7 @@
 
 deps = [
     dependency('libgpiodcxx', default_options: ['bindings=cxx']),
-    dependency('systemd'),
+    dependency('libsystemd'),
     dependency('sdbusplus'),
     dependency('phosphor-logging'),
     dependency('nlohmann_json', include_type: 'system'),