meson: Fix deprecated meson warning

The get_pkgconfig_variable method supports versions below 0.56.0,
and the current meson version is above 0.58.0, so the new method
get_variable should be used.

WARNING: Project targeting '>=0.58.0' but tried to use feature
deprecated since '0.56.0': Dependency.get_pkgconfig_variable. use
Dependency.get_variable(pkgconfig : ...) instead
WARNING: Deprecated features used:
* 0.56.0: {'Dependency.get_pkgconfig_variable'}

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

Signed-off-by: George Liu <liuxiwei@inspur.com>
Change-Id: Ie763577e156382263311ac9a03c4950ccb9594a4
diff --git a/meson.build b/meson.build
index b672b29..9f9c057 100755
--- a/meson.build
+++ b/meson.build
@@ -57,9 +57,9 @@
         install_dir: get_option('bindir'))
 
 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(
     copy: true,