src/poky:flex: Add CVE-2019-6293 to exclusions for checks

CVE is effectively disputed - yes there is stack exhaustion but no bug and it
is building the parser, not running it, effectively similar to a compiler ICE.
Upstream no plans to address and there is no security issue.

westes/flex#414

(From OE-Core rev: 0cae5d7a24bedf6784781b62cbb3795a44bab4d1)

Signed-off-by: Richard Purdie <[email protected]>