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: I98e42b3ac19670c2ff882b03702bffff74957f04
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index b3fe39c..377d296 100644
--- a/meson.build
+++ b/meson.build
@@ -72,7 +72,7 @@
 
 ifacesdir = get_option('IFACES_PATH')
 if ifacesdir == ''
-    ifacesdir = phosphor_dbus_interfaces_dep.get_variable(pkgconfig: 'yamldir', internal: 'yamldir')
+    ifacesdir = phosphor_dbus_interfaces_dep.get_variable('yamldir')
 endif
 
 generated_cpp = custom_target(