meson: Fix deprecated meson warnings

Fix the warnings related to deprecated meson functions:

WARNING: Project targeting '>= 0.57.0' but tried to use feature deprecated since '0.56.0': Dependency.get_pkgconfig_variable. use Dependency.get_variable(pkgconfig : ...) instead

Tested: Verified that there are no such warnings in local CI.

Change-Id: Ie9952c3aceeb6faaed8abe40439bf91e8fc21c2f
Signed-off-by: Pavithra Barithaya <pavithra.b@ibm.com>
diff --git a/meson.build b/meson.build
index 7d94efb..dffc259 100644
--- a/meson.build
+++ b/meson.build
@@ -66,10 +66,9 @@
 # Install the files into the build directory
 
 systemd = dependency ('systemd')
-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(input : 'xyz.openbmc_project.Time.Manager.service',
                output : 'xyz.openbmc_project.Time.Manager.service',
                copy : true,