Install internal hrl files when necessary

Sometimes we install *.erl files. Some these files include a private
*.hrl files, so in order to make these *.erl files usable we have to
install these private includes as well.

Signed-off-by: Peter Lemenkov <lemenkov@gmail.com>
Merge branch 'lemenkov/install_additional_headers/OTP-12197' into maint

* lemenkov/install_additional_headers/OTP-12197:
  Install internal hrl files when necessary
Merge branch 'maint'