CMake has stock FindZLIB in upper case. (#5261)

Summary:
More details in https://cmake.org/cmake/help/v3.14/module/FindZLIB.html

This resolves the cmake config error of not finding `Findzlib` on Linux (CentOS 7 + cmake 3.14.3 + gcc-8).
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5261

Differential Revision: D15138052

Pulled By: maysamyabandeh

fbshipit-source-id: 2f4445f49a36c16e6f1e05c090018c02379c0de4

CMakeLists.txt

@@ -92,7 +92,7 @@ else()
   endif()
 
   if(WITH_ZLIB)
-    find_package(zlib REQUIRED)
+    find_package(ZLIB REQUIRED)
     add_definitions(-DZLIB)
     if(ZLIB_INCLUDE_DIRS)
       # CMake 3