Fix 'implicit-fallthrough' build error on GCC 9.3.0

Fix 'implicit-fallthrough' build error on GCC 9.3.0

It seems the existing comment was not hitting the regular
expression used by the GCC compiler when reading user comments
before case switch fallthrough.

The warning is triggered by -Wextra, which on GCC enables
'-Wimplicit-fallthrough=3'.

Another way to avoid the warning would be to use
[[fallthrough]] statement, but it's from C++17, which does not seem
to be enabled.

Components: Vulkan

Change-Id: Ica52e88f94f4f1c303511d8462938856cb676146
(cherry picked from commit 3cbdbc5564b6684d4737c61d4bcfccd516e14ce7)