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
         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.