Replace deprecated get_pkgconfig_variable with get_variable

- Meson 0.57 warns that get_pkgconfig_variable is deprecated and
  we should be using get_variable instead.

Tested By:

Setup docker, and made sure the service files are installed in the
desired directory.

Installing /home/juliet/Documents/Work/docker/ci_test_area/pldm/build/pldmd.service to /lib/systemd/system
Installing /home/juliet/Documents/Work/docker/ci_test_area/pldm/build/pldm_verbosity to /etc/default
Installing /home/juliet/Documents/Work/docker/ci_test_area/pldm/build/softoff/pldmSoftPowerOff.service to /lib/systemd/system

Signed-off-by: Manojkiran Eda <manojkiran.eda@gmail.com>
Change-Id: I2da6f5fecc37e91eb927b2ebae9bd59de59d1b08
diff --git a/configurations/meson.build b/configurations/meson.build
index 64180cb..99accae 100644
--- a/configurations/meson.build
+++ b/configurations/meson.build
@@ -1,6 +1,3 @@
-systemd_system_unit_dir = systemd.get_pkgconfig_variable(
-    'systemdsystemunitdir',
-    define_variable: ['prefix', get_option('prefix')])
 packagedir = join_paths(
     get_option('prefix'),
     get_option('datadir'),
diff --git a/meson.build b/meson.build
index db22dc0..fdef145 100644
--- a/meson.build
+++ b/meson.build
@@ -133,9 +133,9 @@
   install: true,
   install_dir: get_option('bindir'))
 
-systemd_system_unit_dir = systemd.get_pkgconfig_variable(
-    'systemdsystemunitdir',
-    define_variable: ['prefix', get_option('prefix')])
+systemd_system_unit_dir = systemd.get_variable(
+        pkgconfig: 'systemdsystemunitdir',
+        pkgconfig_define: ['prefix', get_option('prefix')])
 
 configure_file(
   copy: true,
diff --git a/softoff/meson.build b/softoff/meson.build
index 5580d4a..d67a444 100644
--- a/softoff/meson.build
+++ b/softoff/meson.build
@@ -15,10 +15,8 @@
            install: true,
            install_dir: get_option('bindir'))
 
-servicedir = systemd.get_pkgconfig_variable('systemdsystemunitdir')
-
 configure_file(input: 'services/pldmSoftPowerOff.service',
                  output: 'pldmSoftPowerOff.service',
                  copy: true,
-                 install_dir: servicedir)
+                 install_dir: systemd_system_unit_dir)