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.

https://github.com/krb5/krb5/commit/b7a4d695263f1a5b7fe72b1eadce4acdc3f0490b
Author: Ben Kaduk <kaduk@mit.edu>
Commit: b7a4d695263f1a5b7fe72b1eadce4acdc3f0490b
Branch: master
 src/doc/Doxyfile.in |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)