gpgscm: Suppress warnings for GCC > 6.

* tests/gpgscm/scheme.c (CASE): Use unused attribute for GCC > 6.
(FALLTHROUGH): New for fallthrough.
(Eval_Cycle): Use FALLTHROUGH.  Remove not-needed comment of
fallthrough.

Since GCC combines C preprocessor macro expansion, the fallthrough
comment doesn't work well to suppress warnings for
-Wimplicit-fallthrough, near the macro CASE. To handle this
problem, we use GCC's extension of unused label and fallthrough
attributes.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>