vquic: fix compiler warning with gcc + MUSL

[vszakats]

/Users/runner/work/curl-for-win/curl-for-win/curl/lib/vquic/vquic.c: In function 'msghdr_get_udp_gro':
/Users/runner/work/curl-for-win/curl-for-win/curl/lib/vquic/vquic.c:344: warning: ignoring #pragma clang diagnostic [-Wunknown-pragmas]
  344 | #pragma clang diagnostic push
      |
/Users/runner/work/curl-for-win/curl-for-win/curl/lib/vquic/vquic.c:345: warning: ignoring #pragma clang diagnostic [-Wunknown-pragmas]
  345 | #pragma clang diagnostic ignored "-Wsign-compare"
      |
/Users/runner/work/curl-for-win/curl-for-win/curl/lib/vquic/vquic.c:346: warning: ignoring #pragma clang diagnostic [-Wunknown-pragmas]
  346 | #pragma clang diagnostic ignored "-Wcast-align"
      |
/Users/runner/work/curl-for-win/curl-for-win/curl/lib/vquic/vquic.c:350: warning: ignoring #pragma clang diagnostic [-Wunknown-pragmas]
  350 | #pragma clang diagnostic pop
      |

https://github.com/curl/curl-for-win/actions/runs/11356281008/job/31587180874#step:3:9534

Follow-up to a571afc #14012

[dfandrich]

Analysis of PR #15303 at 6ef0e21b:

Test http/test_17_ssl_use.py::TestSSLUse::test_17_09_ssl_min_max[TLSv1-0--2] failed, which has NOT been flaky recently, so there could be a real issue in this PR.

Test http/test_17_ssl_use.py::TestSSLUse::test_17_09_ssl_min_max[TLSv1-0--1] failed, which has NOT been flaky recently, so there could be a real issue in this PR.

Test http/test_17_ssl_use.py::TestSSLUse::test_17_09_ssl_min_max[TLSv1-0-0] failed, which has NOT been flaky recently, so there could be a real issue in this PR.

Test http/test_17_ssl_use.py::TestSSLUse::test_17_09_ssl_min_max[TLSv1-1--2] failed, which has NOT been flaky recently, so there could be a real issue in this PR.

Test http/test_17_ssl_use.py::TestSSLUse::test_17_09_ssl_min_max[TLSv1-1--1] failed, which has NOT been flaky recently, so there could be a real issue in this PR.

Test http/test_17_ssl_use.py::TestSSLUse::test_17_09_ssl_min_max[TLSv1-1-0] failed, which has NOT been flaky recently, so there could be a real issue in this PR.

Test http/test_17_ssl_use.py::TestSSLUse::test_17_09_ssl_min_max[TLSv1-2--2] failed, which has NOT been flaky recently, so there could be a real issue in this PR.

There are more failures, but that's enough from Gha.

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