NFC: Fix implicit fallthrough warning.

Bug: http://b/112564944
Test: Builds.
Change-Id: I3cbb6e751b1341ae8a9abcd04bd16c6b81156ff0
diff --git a/tools/aapt/AaptAssets.cpp b/tools/aapt/AaptAssets.cpp
index 3c3edda..672731c 100644
--- a/tools/aapt/AaptAssets.cpp
+++ b/tools/aapt/AaptAssets.cpp
@@ -301,6 +301,7 @@
                         break;
                     }
                     // This is not alphabetical, so we fall through to variant
+                    [[fallthrough]];
                 case 5:
                 case 6:
                 case 7: