Test on actual libuv version number, not on existence of symbol.

uv_stream_get_write_queue_size isn't a define, it's a
function. It seems like this would always be shadowing the
actual function name.

src/nvim/event/stream.c

@@ -17,7 +17,7 @@
 #endif
 
 // For compatbility with libuv < 1.19.0 (tested on 1.18.0)
-#ifndef uv_stream_get_write_queue_size
+#if UV_VERSION_MINOR < 19
 #define uv_stream_get_write_queue_size(stream) stream->write_queue_size
 #endif