tools/flex: fix PKG_CPE_ID
cpe:/a:westes:flex is the correct CPE ID for flex:
https://nvd.nist.gov/products/cpe/search/results?keyword=cpe:2.3:a:westes:flex
Fixes: c61a2395140d92cdd37d3d6ee43a765427e8e318 (add PKG_CPE_ID ids to package and tools)
Signed-off-by: Fabrice Fontaine <[email protected]>