package/nmap: add zlib optional dependency

zlib is an optional dependency since bump to version 7.60 in commit
c4faf1d4 and
https://github.com/nmap/nmap/commit/0c142333bb356b6ff15fff5c95522c4cfc410a9c.
If it is not disabled, nmap will build its own zlib version which can
result in the following build failure:

/home/giuliobenetti/autobuild/run/instance-1/output-1/host/opt/ext-toolchain/bin/../lib/gcc/i686-buildroot-linux-uclibc/9.3.0/../../../../i686-buildroot-linux-uclibc/bin/ld: attempted static link of dynamic object `libz.so.1.2.11'

Fixes:
 - http://autobuild.buildroot.org/results/da9469e24390c94fe74f133152dc320c21872159
 - http://autobuild.buildroot.org/results/53034d8dd506bc033dc92343f9a37cd4ac8b2142



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>