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: I9768dd66c4180085f201a0b77385d696bdd1680f
diff --git a/meson.build b/meson.build
index 6ec5dca..e244f85 100644
--- a/meson.build
+++ b/meson.build
@@ -140,7 +140,7 @@
 phosphor_logging_dep = dependency('phosphor-logging')
 phosphor_dbus_interfaces_dep = dependency('phosphor-dbus-interfaces')
 sdeventplus_dep = dependency('sdeventplus')
-systemd = dependency('systemd')
+libsystemd_dep = dependency('libsystemd')
 crypto = dependency('libcrypto', version : '>=1.0.2g')
 pam = cpp.find_library('pam', required: true)
 sdbusplus_dep = dependency('sdbusplus')