Frame Relay: denote a genuine fall through case

The source code comment explains it should fall through but GCC does not
pick it up, that's what the new macro is for.

./print-fr.c: In function ‘mfr_print’:
./print-fr.c:510:20: warning: this statement may fall through [-Wimplicit-fallthrough=]
                 if (ie_len == sizeof(struct timeval)) {
                    ^
./print-fr.c:521:13: note: here
             case MFR_CTRL_IE_VENDOR_EXT:
             ^~~~