Suppress -Wimplicit-fallthrough.

-Wimplicit-fallthrough will be enabled globally.

Test: build with global -Wimplicit-fallthrough.
Change-Id: I234ecdd32c9cc8e4b7b8c26e9af7604e66dc3991
diff --git a/Android.bp b/Android.bp
index c7e0da8..81dad26 100644
--- a/Android.bp
+++ b/Android.bp
@@ -41,6 +41,8 @@
     "-std=c++14",
     "-fexceptions",
     "-Wextra",
+    "-Wno-implicit-fallthrough",
+    // src/Unwind-EHABI.cpp line 97 and 100 have implicit fallthrough.
     "-Wno-unused-function",
     "-Wno-unused-parameter",
     "-Wno-unused-local-typedef",