cmake: fix ZLIB_INCLUDE_DIRS use

CMake 2.8's FindZLIB.cmake documents ZLIB_INCLUDE_DIRS, see
http://www.cmake.org/cmake/help/v2.8.0/cmake.html#module:FindZLIB

Bug: https://github.com/bagder/curl/pull/123

CMakeLists.txt

@@ -411,7 +411,7 @@ if(CURL_ZLIB)
     set(HAVE_ZLIB ON)
     set(HAVE_LIBZ ON)
     list(APPEND CURL_LIBS ${ZLIB_LIBRARIES})
-    include_directories(${ZLIB_INCLUDE_DIR})
+    include_directories(${ZLIB_INCLUDE_DIRS})
   endif()
 endif()