RFR: 7903574: jextract should add lang=c to snippets containing C sources

[Weijun Wang]

add the tag when snippet is generated.

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Commit messages:
 - 7903574: jextract should add lang=c to snippets containing C sources

Changes: https://git.openjdk.org/jextract/pull/137/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=137&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903574
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jextract/pull/137.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/137/head:pull/137

PR: https://git.openjdk.org/jextract/pull/137