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: I7c5932b1c6ce342cd3d620dbf1909cf100924ddc
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index d4fb90d..0ae8584 100644
--- a/meson.build
+++ b/meson.build
@@ -1,7 +1,7 @@
 project(
     'phosphor-ledcontroller', 'cpp',
     version : '1.0.0',
-    meson_version: '>=0.57.0',
+    meson_version: '>=0.58.0',
     default_options: [
         'warning_level=3',
         'werror=true',
@@ -19,12 +19,12 @@
     boost,
 ]
 
-udevdir = dependency('udev').get_variable(pkgconfig: 'udevdir')
+udevdir = dependency('udev').get_variable('udevdir')
 install_data(['udev' / 'rules.d' / '70-leds.rules'], install_dir : udevdir / 'rules.d')
 
 systemd = dependency('systemd')
 install_data(['systemd' / 'system' / 'xyz.openbmc_project.led.controller@.service'],
-             install_dir: systemd.get_variable(pkgconfig: 'systemdsystemunitdir')
+             install_dir: systemd.get_variable('systemdsystemunitdir')
 )
 
 sources = [