Add a missing zlib .h file to the src tarball

Fixes building on windows where we use the bundled zlib C sources

Author: dcoutts

zlib.cabal

@@ -20,7 +20,7 @@ build-type:      Simple
 cabal-version:   >= 1.10
 extra-source-files: cbits/crc32.h cbits/inffast.h cbits/inflate.h
                     cbits/trees.h cbits/deflate.h cbits/inffixed.h
-                    cbits/inftrees.h cbits/zutil.h
+                    cbits/inftrees.h cbits/zutil.h cbits/gzguts.h
                     -- test data files
                     test/data/bad-crc.gz test/data/custom-dict.zlib
                     test/data/custom-dict.zlib-dict test/data/hello.gz