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 <hepler@google.com>
Commit-Queue: Ewout van Bekkum <ewout@google.com>
diff --git a/pw_build/BUILD.gn b/pw_build/BUILD.gn
index 339a031..d228f35 100644
--- a/pw_build/BUILD.gn
+++ b/pw_build/BUILD.gn
@@ -73,6 +73,7 @@
     "-Wimplicit-fallthrough",
     "-Wcast-qual",
     "-Wundef",
+    "-Wpointer-arith",
 
     # Make all warnings errors, except for the exemptions below.
     "-Werror",