Map .hin files to the C language for doxygen

In Debian unstable, the current version of doxygen is unhappy with
our generated Doxyfile, and does not handle krb5.hin in the expected
fashion (as a C header).  Work around this issue by explicitly
specifying that files with the .hin extension are to be treated
as C language files.

ticket: 8004 (new)
tags: pullup
target_version: 1.13