Remove circular dependencies in regulator includes

Remove the circular dependencies that currently exist among regulator
include files.  Add forward declarations in two key includes that cause
the issue.

This requires the creation of a new source (.cpp) file and as a result a
new static library.

Change-Id: Ib136adf0a0cf6b30781b64cfecede0fbd50e13f7
Signed-off-by: Shawn McCarney <shawnmm@us.ibm.com>
diff --git a/phosphor-regulators/src/meson.build b/phosphor-regulators/src/meson.build
index 23266b0..76551ae 100644
--- a/phosphor-regulators/src/meson.build
+++ b/phosphor-regulators/src/meson.build
@@ -2,3 +2,14 @@
     '.',
     'actions'
 )
+
+phosphor_regulators_source_files = [
+    'id_map.cpp'
+]
+
+phosphor_regulators_library = static_library(
+    'phosphor-regulators',
+    phosphor_regulators_source_files,
+    implicit_include_directories: false,
+    include_directories: phosphor_regulators_include_directories
+)