pw_build: add "-Wpointer-arith" to strict_warnings
pw_build: add "-Wpointer-arith" to strict_warnings

Adds "-Wpointer-arith" to "strict_warnings" to catch anything that
depends on the “size of” a function type or void.

Change-Id: I3d83c7a9c3c8c438c06592637c3c541c5378eac6
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/21684
Reviewed-by: Wyatt Hepler <cca3bedb37979eacd157a9bceb473582df8aea1e@google.com>
Commit-Queue: Ewout van Bekkum <638f134dbc68778d6b859fb04a64baea2b00b021@google.com>

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed