Create regulators StubAction class

Created the StubAction class to aid in unit testing the
phosphor-regulators application.  This simple 'stub' will either return
a predefined return value or throw a predefined exception.

Updated the meson.build file to search for includes in the test
tree.  This is where stub classes will be located.

Signed-off-by: Shawn McCarney <shawnmm@us.ibm.com>
Change-Id: I78030d29f3bc6a94db998c5a1712a3b4a03c3e90
diff --git a/phosphor-regulators/src/meson.build b/phosphor-regulators/src/meson.build
index 19f1bb5..23266b0 100644
--- a/phosphor-regulators/src/meson.build
+++ b/phosphor-regulators/src/meson.build
@@ -1 +1,4 @@
-# subdir('actions')
+phosphor_regulators_include_directories = include_directories(
+    '.',
+    'actions'
+)