Makefile: Suppress `-Wimplicit-fallthrough` when compiling `lex.yy`

Makefile: Suppress `-Wimplicit-fallthrough` when compiling `lex.yy`

lex.yy.c is an auto generated C file. When compiling with clang-15, we
got the following warning:

```
      CC lex.yy.o
  lex.yy.c:1444:5: warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]
                                  case EOB_ACT_END_OF_FILE:
                                  ^
  lex.yy.c:1444:5: note: insert '__attribute__((fallthrough));' to silence this warning
                                  case EOB_ACT_END_OF_FILE:
                                  ^
                                  __attribute__((fallthrough));
  lex.yy.c:1444:5: note: insert 'break;' to avoid fall-through
                                  case EOB_ACT_END_OF_FILE:
                                  ^
                                  break;
  1 warning generated.
```

There is nothing we can do to fix lex.yy.c since it's an auto generated
file. Fix this by appending `-Wno-implicit-fallthrough` when compiling
this file if we have `-Wimplicit-fallthrough` flag enabled.

Reviewed-by: Niklas Cassel <niklas.cassel@wdc.com>
Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
Link: https://lore.kernel.org/r/20220512164333.46516-4-ammarfaizi2@gnuweeb.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>