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>
(cherry picked from commit 7a94fa901293bda6e0e87eff5dcb677b856c70b4)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/libzlib/libzlib.mk

@@ -11,7 +11,7 @@ LIBZLIB_LICENSE = Zlib
 LIBZLIB_LICENSE_FILES = README
 LIBZLIB_INSTALL_STAGING = YES
 LIBZLIB_PROVIDES = zlib
-LIBZLIB_CPE_ID_VENDOR = gnu
+LIBZLIB_CPE_ID_VENDOR = zlib
 LIBZLIB_CPE_ID_PRODUCT = zlib
 
 # It is not possible to build only a shared version of zlib, so we build both