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
WARNING: Project targeting '>= 0.57.0' but tried to use feature deprecated since '0.56.0': meson.source_root. use meson.project_source_root() or meson.global_source_root() instead.

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

Signed-off-by: Lei YU <yulei.sh@bytedance.com>
Change-Id: I1388aaa12161d020d06657e507ddef78cb116ffe
diff --git a/meson.build b/meson.build
index 1367a72..f6f3f35 100644
--- a/meson.build
+++ b/meson.build
@@ -150,7 +150,7 @@
 ssl = dependency('openssl')
 
 systemd = dependency('systemd')
-systemd_system_unit_dir = systemd.get_pkgconfig_variable('systemdsystemunitdir')
+systemd_system_unit_dir = systemd.get_variable(pkgconfig : 'systemdsystemunitdir')
 
 unit_files = [
     'obmc-flash-bmc-setenv@.service.in',