The earlier changes to ensure all header only files are installed was
causing issues with the wrappers, introduced a new variable that can be
set to specify additional header files that should be installed called
[module name]_HDRS, which is used in one or two places.

Note, it should normally be enough to simply mark the header as ABSTRACT
or WRAP_EXCLUDE.

Change-Id: I8edede84b5f23a2d0fa47cdc88e3beab293d7180