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: I1c870674caf5e31890732c0aec7c4d69751bb604
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index 048cbce..6d395ed 100644
--- a/meson.build
+++ b/meson.build
@@ -1,7 +1,7 @@
 project(
     'openpower-occ-control', 'cpp',
     version : '1.0.0',
-    meson_version: '>=0.57.0',
+    meson_version: '>=0.58.0',
     default_options: [
         'warning_level=3',
         'werror=true',
@@ -55,7 +55,7 @@
 
 systemd = dependency('systemd')
 systemd_system_unit_dir = systemd.get_variable(
-    pkgconfig : 'systemdsystemunitdir')
+    'systemdsystemunitdir')
 subdir('service_files')
 
 sdbusplus_dep = dependency('sdbusplus')