libc: enable -Wimplicit-fallthrough

libc had some -Wimplicit-fallthrough warnings. They all seem to be
benign. We're trying to enable this flag globally, so we need to
annotate these breaks here.

Bug: 112564944
Test: Builds
Change-Id: I5afae694cc4cf26ad1a61e2c8ae91f00cda7c733
diff --git a/libc/Android.bp b/libc/Android.bp
index 1f95ce8..7356c64 100644
--- a/libc/Android.bp
+++ b/libc/Android.bp
@@ -41,6 +41,7 @@
     "-Wno-deprecated-declarations",
     "-Wno-gcc-compat",
     "-Wframe-larger-than=2048",
+    "-Wimplicit-fallthrough",
 
     // Try to catch typical 32-bit assumptions that break with 64-bit pointers.
     "-Werror=pointer-to-int-cast",