libpayload: Enable -Wimplicit-fallthrough

Add comments to intentional fall throughs and enable the warning.

Change-Id: I93e071c4fb139fa6e9cd8a1bfb5800f5f4eac50b
Signed-off-by: Jacob Garber <jgarber1@ualberta.ca>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/34457
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: HAOUAS Elyes <ehaouas@noos.fr>

payloads/libpayload/curses/PDCurses/pdcurses/scanw.c

@@ -274,7 +274,7 @@ static int _pdc_vsscanf(const char *buf, const char *fmt, va_list arg_ptr)
                             NEXT(c);
                             goto string;
                         }
-                        /* no break */
+                        /* fall through */
                     default:
                         if (fmt[1] == '-' && fmt[2]
                             && f < (unsigned char)fmt[2])