GCC: Fix zlib symbol visibility macro

GCC parses the function __attribute__ syntax a bit differently from Clang,
associating the attribute with the return type instead of the function if the
return type is a pointer. This leads to certain zlib symbols, such as
Cr_z_zError, to fail to be properly exported from the shared library. Fix by
using ZEXTERN instead of ZEXPORT for the attribute which works the same for both
GCC and Clang.

Bug: 819294
Change-Id: I5707d1b0627a503df08df9ac0bb65fda69453989
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1859788


Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#705917}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 9c3aed099b010a75594a0efd523774c4c9a5e3d2