Add .h.in to C

[donno2048]

From line 4256 (iirc) we know linguist does recognize dynamically created files

Checklist:

• I am adding a new extension to a language.
  • The new extension is used in hundreds of repositories on GitHub.com
    • Search results for each extension:
      • https://github.com/search?type=code&q=NOT+is%3Afork+path%3A*.h.in+include
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • CEN64 github page ( https://github.com/n64dev/cen64/blob/master/common.h.in)
    • Sample license(s): BSD 3-Clause

[lildude]

Please follow the directions for adding a new extension as detailed in the CONTRIBUTING.md file and please put back and complete the "I am adding a new extension to a language." section of the PR template you have removed - it's there for a reason.

[donno2048]

@lildude did you mean adding a sample? If so, I added one and updated the PR comment

[lildude]

If these files are dynamically created, would it make sense to mark them as generated as part of this PR? This means they will be suppressed in PR diffs and excluded from the stats by default.

[donno2048]

@lildude they're used to generate .h files, kind of a header templating format, so it wouldn't make sense to mark them as generated, they're more "generating" files...

[lildude]

LGTM. Thanks.

Important

The changes in this PR will not appear on GitHub until the next release has been made and deployed. See here for more details.