Fix a build failure where recvmmsg is available but not sendmmsg #876

Job	Run time
basic_clang	7m 11s
address_ub_sanitizer	17m 28s
basic_gcc	7m 34s
buildtest	12m 33s
check_docs	1m 15s
check_update	1m 39s
check-ansi	4m 24s
enable_brotli_and_zstd_dynamic	8m 25s
enable_brotli_and_asan_ubsan	18m 34s
enable_brotli_dynamic	8m 58s
enable_zstd_and_asan_ubsan	13m 19s
enable_non-default_options	6m 36s
enable_zstd_dynamic	8m 49s
enable-tfo (ubuntu-latest)	7m 10s
enable-tfo (macos-latest)	11m 14s
external-test-cf-quiche	7m 17s
external-test-pyca (1.51.0, 3.9)	13m 48s
external-tests	13m 59s
legacy	5m 43s
fips_and_ktls	9m 0s
memory_sanitizer	19m 37s
no-deprecated	8m 45s
minimal	4m 8s
no-legacy	9m 51s
non-caching	23m 32s
no-shared (ubuntu-latest)	6m 20s
no-shared (macos-latest)	8m 21s
out-of-source-and-install (ubuntu-latest)	11m 19s
out-of-source-and-install (macos-latest)	16m 29s
threads_sanitizer	6m 12s
	4h 59m 30s

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