CI failing on macOS: -Werror,-Wmissing-field-initializers

macOS CI for !3069 (merged) failed with:

FAILED: gthread/tests/init.p/init.c.o 
cc -Igthread/tests/init.p -Igthread/tests -I../gthread/tests -I. -I.. -Iglib -I../glib -Isubprojects/proxy-libintl -I../subprojects/proxy-libintl -fcolor-diagnostics -Wall -Winvalid-pch -Wextra -Wpedantic -Werror -std=gnu99 -O0 -g -D_GNU_SOURCE -fno-strict-aliasing -DG_ENABLE_DEBUG -Wimplicit-fallthrough -Wunused -Wno-unused-parameter -Wno-pedantic -Wno-format-zero-length -Wno-variadic-macros -Werror=format=2 -Werror=init-self -Werror=missing-include-dirs -Werror=pointer-arith -Werror=unused-result -Wstrict-prototypes -Wno-bad-function-cast -Werror=implicit-function-declaration -Werror=missing-prototypes -Werror=pointer-sign -Wno-string-plus-int -Wno-overlength-strings '-DG_LOG_DOMAIN="GLib-GThread"' -UG_DISABLE_ASSERT -MD -MQ gthread/tests/init.p/init.c.o -MF gthread/tests/init.p/init.c.o.d -o gthread/tests/init.p/init.c.o -c ../gthread/tests/init.c
../gthread/tests/init.c:33:46: error: missing field 'mutex_lock' initializer [-Werror,-Wmissing-field-initializers]
  const GThreadFunctions functions = { NULL, };
                                             ^

This probably needs a -Wno-missing-field-initializers -Wno-error=missing-field-initializers.