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: I1ec9e9a8efcfe37f0439a15e986bf6e7a754dc00
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index 8debf27..a102da5 100644
--- a/meson.build
+++ b/meson.build
@@ -35,7 +35,7 @@
 
 systemd = dependency('systemd')
 systemd_system_unit_dir = systemd.get_variable(
-        pkgconfig: 'systemdsystemunitdir',
+        'systemdsystemunitdir',
         pkgconfig_define: ['prefix', get_option('prefix')])
 
 configure_file(input: 'phosphor-pid-control.service.in',