meson: remove deprecated get_pkgconfig_variable

Since meson 0.56, the `get_pkgconfig_variable` has been deprecated.  In
meson 0.58 the `get_variable` was enhanced to no longer require the
`pkgconfig` keyword argument.  Ensure meson 0.58 is required and update
the usage of all `get_pkgconfig_variable` and `get_variable` to be the
modern variant.

Change-Id: I16a6ed1a6d4dfdd367464a087fa0f3d23b0d588f
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/service_files/meson.build b/service_files/meson.build
index 6b4dd8c..1a74ec9 100644
--- a/service_files/meson.build
+++ b/service_files/meson.build
@@ -6,4 +6,4 @@
   output: 'xyz.openbmc_project.bios_bmc_smm_error_logger.service',
   configuration: conf_data,
   install: true,
-  install_dir: systemd.get_variable(pkgconfig: 'systemdsystemunitdir'))
+  install_dir: systemd.get_variable('systemdsystemunitdir'))