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: I8599189ddf9cdcbb564252d29244101ee9bd292d
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index 23c92c2..c9072e7 100644
--- a/meson.build
+++ b/meson.build
@@ -7,7 +7,7 @@
     ],
     license: 'Apache-2.0',
     version: '0.1',
-    meson_version: '>=0.57.0',
+    meson_version: '>=0.58.0',
 )
 
 add_project_arguments(
@@ -59,7 +59,7 @@
     'service_files/chassis-system-reset.service',
     'service_files/chassis-system-reset.target',
     'service_files/xyz.openbmc_project.Chassis.Control.Power@.service',
-    install_dir: systemd.get_variable(pkgconfig: 'systemdsystemunitdir'))
+    install_dir: systemd.get_variable('systemdsystemunitdir'))
 endif
 
 install_data(