nios2: signal: Mark expected switch fall-through

Mark switch cases where we are expecting to fall through.

Fix the following warning through the use of the new the new
pseudo-keyword fallthrough;

arch/nios2/kernel/signal.c:254:12: warning: this statement may fall through [-Wimplicit-fallthrough=]
  254 |    restart = -2;
      |    ~~~~~~~~^~~~
arch/nios2/kernel/signal.c:255:3: note: here
  255 |   case ERESTARTNOHAND:
      |   ^~~~

Reported-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Ley Foon Tan <ley.foon.tan@intel.com>