checkpatch: exclude POSIX headers from typedef rule

The `NEW_TYPEDEFS` rule consistently generates false
positives when implementing POSIX standard APIs.

It makes sense to disable this check for the POSIX haders
rather than requiring merge superpowers constantly. That
way, we can merge as per usual after sufficient approvals
rather than waiting for someone with merge superpowers to
override / manually merge.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>