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: I46c094c1a1e4793556e93f9e5158bff7c6042cc4
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/side-switch/meson.build b/side-switch/meson.build
index b13e53c..9d6cbf0 100644
--- a/side-switch/meson.build
+++ b/side-switch/meson.build
@@ -15,7 +15,7 @@
 )
 
 systemd_system_unit_dir = dependency('systemd').get_variable(
-    pkgconfig: 'systemdsystemunitdir',
+    'systemdsystemunitdir',
     pkgconfig_define: ['prefix', get_option('prefix')])
 
 install_data(