Disable warning for missing-field-initializers as well for clang.

The same warning shows up with clang++. Tested with clang-16 and clang-17.

Change-Id: I4181f7eb20a17b83aef1d4d0c4a34d42818f53a1
Reviewed-on: https://code-review.googlesource.com/c/re2/+/62730
Reviewed-by: Alex Chernyakhovsky <achernya@google.com>
Reviewed-by: Paul Wankadia <junyer@google.com>