package/libzlib: fix CPE vendor id

gnu vendor is not found for zlib in CPE database [1].
zlib is the correct vendor [2].

[1] https://nvd.nist.gov/products/cpe/search/results?namingFormat=2.3&keyword=cpe:2.3:a:gnu:zlib
[2] https://nvd.nist.gov/products/cpe/search/results?namingFormat=2.3&keyword=cpe:2.3:a:zlib:zlib



Signed-off-by: Marcus Hoffmann <marcus.hoffmann@othermo.de>
Signed-off-by: Arnout Vandecappelle <arnout@mind.be>