build: Remove deprecated get_pkgconfig_variable method

This also removes the definition of prefix, it does nothing in the
content of systemd.pc and would wrongly prepend /usr if it was hooked
up in the pc file.

Change-Id: I44d788753e2f1b7840afaea89648ed0b572a622f
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/dist/meson.build b/dist/meson.build
index a38f80b..6c82a3e 100644
--- a/dist/meson.build
+++ b/dist/meson.build
@@ -1,15 +1,14 @@
-systemd_system_unit_dir = dependency('systemd').get_pkgconfig_variable(
-    'systemdsystemunitdir',
-    define_variable: ['prefix', get_option('prefix')])
+systemd_system_unit_dir = dependency('systemd').get_variable(
+    pkgconfig: 'systemdsystemunitdir')
 
 install_data(
     ['xyz.openbmc_project.Logging.service',
      'xyz.openbmc_project.Syslog.Config.service'],
-    install_dir: systemd_system_unit_dir
+    install_dir: systemd_system_unit_dir,
 )
 
 install_data(
-    ['busconfig/phosphor-logging.conf',
-     'busconfig/phosphor-rsyslog-config.conf'],
-    install_dir: get_option('datadir') / 'dbus-1' / 'system.d'
+    'busconfig/phosphor-logging.conf',
+    'busconfig/phosphor-rsyslog-config.conf',
+    install_dir: get_option('datadir') / 'dbus-1' / 'system.d',
 )