pw_build: Add "-Wimplicit-fallthrough" to strict_warnings

Enables "-Wimplicit-fallthrough" as part of the
"strict_warnings" config to ensure only explicit fallthrough is
permitted.

Change-Id: If72d56975f460ea70fa5275bcb592e593c26f986
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/21462
Commit-Queue: Ewout van Bekkum <ewout@google.com>
Reviewed-by: Wyatt Hepler <hepler@google.com>
diff --git a/pw_build/BUILD.gn b/pw_build/BUILD.gn
index d02cde1..dfaad82 100644
--- a/pw_build/BUILD.gn
+++ b/pw_build/BUILD.gn
@@ -70,6 +70,7 @@
   cflags = [
     "-Wall",
     "-Wextra",
+    "-Wimplicit-fallthrough",
 
     # Make all warnings errors, except for the exemptions below.
     "-Werror",