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: I2b3f8d8b89bfd555062b75d523c4ecd76e1b87c1
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index 8e6ac29..1b0f3e2 100644
--- a/meson.build
+++ b/meson.build
@@ -2,7 +2,7 @@
   'phosphor-sel-logger',
   'cpp',
   version: '0.1',
-  meson_version: '>=0.57.0',
+  meson_version: '>=0.58.0',
   default_options: [
     'werror=true',
     'warning_level=3',
@@ -54,5 +54,5 @@
 if systemd.found()
   install_data(
     'service_files/xyz.openbmc_project.Logging.IPMI.service',
-    install_dir: systemd.get_variable(pkgconfig: 'systemdsystemunitdir'))
+    install_dir: systemd.get_variable('systemdsystemunitdir'))
 endif