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: I4bf91f3c9a61c6f0c4f14f591ed00a309b7612e6
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/dist/meson.build b/dist/meson.build
index 378d4fc..67be46a 100644
--- a/dist/meson.build
+++ b/dist/meson.build
@@ -1,5 +1,5 @@
 systemd_system_unit_dir = dependency('systemd').get_variable(
-    pkgconfig: 'systemdsystemunitdir')
+    'systemdsystemunitdir')
 
 install_data(
     ['xyz.openbmc_project.Logging.service',
@@ -14,7 +14,7 @@
 )
 
 dbus_system_bus_services_dir = dependency('dbus-1').get_variable(
-    pkgconfig: 'system_bus_services_dir',
+    'system_bus_services_dir',
     pkgconfig_define: ['prefix', get_option('prefix')])
 install_data(
     'dbus/xyz.openbmc_project.Logging.service',