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.

Change-Id: Ia48b6da8923f2955e88819120f1bfc464fec2c97
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index 33eadae..5dec932 100644
--- a/meson.build
+++ b/meson.build
@@ -79,6 +79,7 @@
 endif
 
 systemd = dependency('systemd')
+libsystemd_dep = dependency('libsystemd')
 servicedir = systemd.get_variable('systemdsystemunitdir')
 
 services = [