meson: Fix deprecated meson warning

The get_pkgconfig_variable method supports versions below 0.56.0,
and the current meson version is above 0.57.0, so the new method
get_variable should be used.

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
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: Id328a9e3b7a22892c7d5d94be3241bbc7b91df4c
diff --git a/meson.build b/meson.build
index 5ff07fb..698aa7d 100644
--- a/meson.build
+++ b/meson.build
@@ -93,4 +93,4 @@
   output: 'phosphor-virtual-sensor.service',
   configuration: conf_data,
   install: true,
-  install_dir: systemd.get_pkgconfig_variable('systemdsystemunitdir'))
+  install_dir: systemd.get_variable(pkgconfig: 'systemdsystemunitdir'))