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: I095be438ae99d7bf26d3cbc49de1d4947684856a
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index 2e4240e..533cf8f 100644
--- a/meson.build
+++ b/meson.build
@@ -2,7 +2,7 @@
   'ipmbbridge',
   'cpp',
   version: '0.1',
-  meson_version: '>=0.57.0',
+  meson_version: '>=0.58.0',
   default_options: [
     'warning_level=3',
     'werror=true',
@@ -61,7 +61,7 @@
 
 install_data(
   'ipmb.service',
-  install_dir: systemd_dep.get_variable(pkgconfig: 'systemdsystemunitdir'),
+  install_dir: systemd_dep.get_variable('systemdsystemunitdir'),
 )
 
 install_data(