regulators: Add configure support to Rail class

Implemented the configure() method in the Rail class.  This method
applies any configuration changes that are defined for the rail.

Also added a few missing #includes in the configuration_tests.cpp file.

Signed-off-by: Shawn McCarney <shawnmm@us.ibm.com>
Change-Id: I4164621f6d599bbf1417ebb9328a35bd2b2d656e
diff --git a/phosphor-regulators/src/meson.build b/phosphor-regulators/src/meson.build
index ef4bec8..7415050 100644
--- a/phosphor-regulators/src/meson.build
+++ b/phosphor-regulators/src/meson.build
@@ -12,6 +12,7 @@
     'exception_utils.cpp',
     'id_map.cpp',
     'pmbus_utils.cpp',
+    'rail.cpp',
     'system.cpp',
 
     'actions/if_action.cpp',