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: I85d8e7d41c3fbcf6f21533d1a95d73959ca877e4
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index 9dd5fea..170655d 100644
--- a/meson.build
+++ b/meson.build
@@ -9,7 +9,7 @@
        'buildtype=debugoptimized'
      ],
      version: '1.0',
-     meson_version: '>=0.57.0',
+     meson_version: '>=0.58.0',
 )
 
 add_global_arguments('-Wno-psabi', language : ['c', 'cpp'])
@@ -159,10 +159,10 @@
 endif
 
 systemd_system_unit_dir = dependency('systemd').get_variable(
-        pkgconfig: 'systemdsystemunitdir')
+        'systemdsystemunitdir')
 
 udev_dir = dependency('udev').get_variable(
-        pkgconfig: 'udev_dir')
+        'udev_dir')
 
 install_data(
     'ibm_vpd/system-vpd.service',