build/meson: always set SESSION_TRACKING_SYSTEMD define

Like we do with autotools, otherwise we get a warning:

    [576/1030] Compiling C object src/25a6634@@NetworkManager@sta/nm-session-monitor.c.o.
    ../src/nm-session-monitor.c:31:5: warning: "SESSION_TRACKING_SYSTEMD" is not defined, evaluates to 0 [-Wundef]
     #if SESSION_TRACKING_SYSTEMD && SESSION_TRACKING_ELOGIND
         ^~~~~~~~~~~~~~~~~~~~~~~~
9 jobs for master in 17 minutes and 58 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #99049
allowed to fail
checkpatch

00:01:08

passed #99050
f28_build_autotools

00:07:44

passed #99051
f28_build_meson

00:05:07

passed #99052
f28_rpm_autotools_dbg

00:08:04

passed #99053
f28_rpm_meson_dbg

00:10:12

passed #99054
f_latest_build_autotools

00:17:37

failed #99055
allowed to fail
f_rawhide_build_autotools

00:07:05

 
  Deploy
passed #99056
pages

00:00:19

failed pages:deploy

00:00:00

 
Name Stage Failure
failed
f_rawhide_build_autotools Test
  CC       src/devices/wwan/libnm_device_plugin_wwan_la-nm-device-modem.lo
cc1: all warnings being treated as errors
make[2]: *** [Makefile:14560: src/devices/wwan/libnm_wwan_la-nm-modem-broadband.lo] Error 1
make[2]: *** Waiting for unfinished jobs....
make[2]: Leaving directory '/builds/NetworkManager/NetworkManager/build'
make[1]: *** [Makefile:18308: all-recursive] Error 1
make[1]: Leaving directory '/builds/NetworkManager/NetworkManager/build'
make: *** [Makefile:8167: all] Error 2
ERROR: Job failed: exit code 1