Move to subproject to enable both projects

Yocto builds need each component to build individually, but meson wants
them to build as a single project. This follows google-misc and moves
to subprojects.

It also enables callback-manager in the build and fixes some build
errors and warnings.

Change-Id: Ie56141bf86b6d9c6b27eb697944fbc392e374c22
Signed-off-by: Jason M. Bills <jason.m.bills@linux.intel.com>
diff --git a/meson.build b/meson.build
index 99c3ca7..96ed4ef 100644
--- a/meson.build
+++ b/meson.build
@@ -7,4 +7,5 @@
 )
 
 #Add subproject for hsbp-manager
-subdir('hsbp-manager')
+subproject('hsbp-manager')
+subproject('callback-manager')