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: I5c96e289619a9677867d3fcf4a7b690d370b80c2
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index f37ac3d..c32e295 100644
--- a/meson.build
+++ b/meson.build
@@ -10,7 +10,7 @@
     ],
     license: 'Apache-2.0',
     version: '1.0',
-    meson_version: '>=0.57.0',
+    meson_version: '>=0.58.0',
 )
 
 build_tests = get_option('tests')
@@ -46,7 +46,7 @@
 libgpiodcxx = dependency('libgpiodcxx')
 
 systemd = dependency('systemd')
-servicedir = systemd.get_variable(pkgconfig : 'systemdsystemunitdir')
+servicedir = systemd.get_variable('systemdsystemunitdir')
 
 services = [
     ['supply-monitor', 'power-supply-monitor@.service'],