deps: fix zlib -Wimplicit-function-declaration

Build the bundled zlib with -DZ_HAVE_UNISTD_H to make the definition of
close(), read() and other unistd.h functions available to gzread.c and
gzwrite.c. It's kind of silly that we have to jump through hoops here
because we never call any of the functions that do I/O directly, but at
least it squelches the -Wimplicit-function-declaration warnings.

PR-URL: #273
Reviewed-by: Bert Belder <bertbelder@gmail.com>

deps/zlib/zlib.gyp

@@ -62,6 +62,7 @@
             ['OS!="win"', {
               'product_name': 'chrome_zlib',
               'cflags!': [ '-ansi' ],
+              'defines': [ 'Z_HAVE_UNISTD_H' ],
               'sources!': [
                 'contrib/minizip/iowin32.c'
               ],