flex: Make sure scan.c is newer than scan.l
flex: Make sure scan.c is newer than scan.l

Sometimes, flex fails to build with error:
  | WARNING: 'flex' is missing on your system
  |          You should only need it if you modified a '.l' file.
  |          You may want to install the Fast Lexical Analyzer package
  |          <https://github.com/westes/flex>
  | make[2]: *** [Makefile:1462: scan.c] Error 127

Update timestamp of scan.c to make sure it is newer than scan.l.

Signed-off-by: Trung Do <64081e6b7eb2b24289b8a4e51cef9836ddf28af2@toshiba-tsdv.com>

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