mark fall-through in switch-case to calm linter

src/pk/ecc/ecc_import_pkcs8.c

@@ -73,10 +73,15 @@ static int _simple_utf8_to_utf16(const unsigned char *in, unsigned long inlen,
       if (in + extra >= in_end) goto ERROR;
       switch (extra) {
          case 5: ch += *in++; ch <<= 6;
+         /* FALLTHROUGH */
          case 4: ch += *in++; ch <<= 6;
+         /* FALLTHROUGH */
          case 3: ch += *in++; ch <<= 6;
+         /* FALLTHROUGH */
          case 2: ch += *in++; ch <<= 6;
+         /* FALLTHROUGH */
          case 1: ch += *in++; ch <<= 6;
+         /* FALLTHROUGH */
          case 0: ch += *in++;
       }
       ch -= offset[extra];