Add category to zlib as packagename is ambiguous

Fixes the following problem:

    [ Results for search key : zlib ]
    [ Applications found : 2 ]

    *  dev-haskell/zlib
          Latest version available: 0.5.3.3
          Latest version installed: 0.5.3.3
          Size of files: 122 kB
          Homepage:      http://hackage.haskell.org/package/zlib
          Description:   Compression and decompression in the gzip and zlib formats
          License:       BSD

    *  sys-libs/zlib
          Latest version available: 1.2.7
          Latest version installed: 1.2.7
          Size of files: 547 kB
          Homepage:      http://www.zlib.net/
          Description:   Standard (de)compression library
          License:       ZLIB

    !!! The short ebuild name "zlib" is ambiguous. Please specify
    !!! one of the above fully-qualified ebuild names instead.
Merge pull request #1599 from xhochy/fix/gentoo-ambiguous-zlib

Add category to zlib as packagename is ambiguous