meson: fix build failure when -Dsystemdsystemunitdir is specified

When 'systemdsystemunitdir' is specified as an option the systemd_dep
variable is not defined, but the sysusers.d directory lookup uses it,
causing a build failure:

dh_auto_configure -- \
	-Dexamples=false \
	-Dintrospection=true \
	-Dman=true \
	-Dsystemdsystemunitdir=/usr/lib/systemd/system \
	-Dtests=true \
	-Dgtk_doc=true -Dsession_tracking=libsystemd-login
	cd obj-x86_64-linux-gnu && DEB_PYTHON_INSTALL_LAYOUT=deb LC_ALL=C.UTF-8 meson setup .. --wrap-mode=nodownload --buildtype=plain --prefix=/usr --sysconfdir=/etc --localstatedir=/var --libdir=lib/x86_64-linux-gnu -Dpython.bytecompile=-1 -Dexamples=false -Dintrospection=true -Dman=true -Dsystemdsystemunitdir=/usr/lib/systemd/system -Dtests=true -Dgtk_doc=true -Dsession_tracking=libsystemd-login
The Meson build system
Version: 1.3.1
Source dir: /builds/bluca/polkit/debian/output/source_dir
Build dir: /builds/bluca/polkit/debian/output/source_dir/obj-x86_64-linux-gnu
Build type: native build
Project name: polkit
Project version: 124

<...>

Run-time dependency libsystemd found: YES 255
Checking for function "sd_uid_get_display" with dependency libsystemd: YES
Checking for function "sd_pidfd_get_session" with dependency libsystemd: YES
../meson.build:222:37: ERROR: Unknown variable "systemd_dep".

Follow-up for 24f1e0a

meson.build

@@ -212,9 +212,9 @@ if enable_logind
   config_h.set10('HAVE_' + func.to_upper(), cc.has_function(func, dependencies: logind_dep))
 
   # systemd unit / service files
+  systemd_dep = dependency('systemd', not_found_message: 'systemd required but not found, please provide a valid systemd user unit dir or disable it')
   systemd_systemdsystemunitdir = get_option('systemdsystemunitdir')
   if systemd_systemdsystemunitdir == '' and session_tracking == 'libsystemd-login'
-    systemd_dep = dependency('systemd', not_found_message: 'systemd required but not found, please provide a valid systemd user unit dir or disable it')
     # FIXME: systemd.pc file does not use variables with relative paths, so `define_variable` cannot be used
     systemd_systemdsystemunitdir = systemd_dep.get_pkgconfig_variable('systemdsystemunitdir')
   endif