Introduce the DEPENDMODULE_HEADERS variable.

Reviewer: @jnealtowns

Modules that require the public headers of another module in order to compile
but don't want to compile that module itself can now add those modules to
the DEPENDMODULE_HEADERS variable.

The current version is not entirely correct as it grabs the $(mod)/module/inc
directory directly when it should probably honor the $(mod)_INCLUDES variable
exported by the module's own makefiles. That setting would have to be
teased out of the makefile structure first.