[cflags] Add -Wno-extra-semi

boringssl has some extra semicolons after macros that add semicolons.
For instance:
WEAK_SYMBOL_FUNC(void, sdallocx, (void *ptr, size_t size, int flags));
The above results in two trailing semis.

Bug: 56257

Change-Id: If0c32ae476e4da7a9cdbb2b9921144f46b18ec41
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/boringssl/+/600342
Reviewed-by: Tamir Duberstein <tamird@google.com>
Fuchsia-Auto-Submit: Shai Barack <shayba@google.com>
Commit-Queue: Shai Barack <shayba@google.com>