Use sourceforge url to fetch zlib (#2297)

Summary:
This PR addresses https://github.com/pytorch/audio/issues/2295 by updating `zlib`'s url to the one on sourceforge.net.
`zlib` 1.2.11 source code is removed from the official site. According to https://zlib.net, ```Due to the bug fixes, any installations of 1.2.11 should be replaced with 1.2.12.```
sourceforge preserves the older versions thus is more stable. The PR keep 1.2.11 as currently there is no 1.2.12 on sourceforge.

Pull Request resolved: https://github.com/pytorch/audio/pull/2297

Reviewed By: mthrok

Differential Revision: D35251361

Pulled By: nateanl

fbshipit-source-id: 174c2c2e1c34bef9799bbacfd1e12c8ff13ff15d

third_party/zlib/CMakeLists.txt

@@ -22,7 +22,7 @@ set(
 ExternalProject_Add(zlib-
   PREFIX ${CMAKE_CURRENT_BINARY_DIR}
   DOWNLOAD_DIR ${ARCHIVE_DIR}
-  URL https://zlib.net/zlib-1.2.11.tar.gz
+  URL https://sourceforge.net/projects/libpng/files/zlib/1.2.11/zlib-1.2.11.tar.gz
   URL_HASH SHA256=c3e5e9fdd5004dcb542feda5ee4f0ff0744628baf8ed2dd5d66f8ca1197cb1a1
   BUILD_BYPRODUCTS ${ZLIB_LIBRARIES}
   CONFIGURE_COMMAND ${CMAKE_COMMAND} -E env ${envs} ${CMAKE_CURRENT_BINARY_DIR}/src/zlib-/configure --static