Hide all symbols by default

As mentioned by @abique. Could prevent issues caused by symbol clashes,
and it also greatly reduces the size of all binaries.

CHANGELOG.md

@@ -13,6 +13,8 @@ Versioning](https://semver.org/spec/v2.0.0.html).
 - When `YABRIDGE_DEBUG_LEVEL` is set to 2 or higher and a plugin asks the host
   for the current position in the song, yabridge will now print that position in
   quarter notes and samples as part of the debug output.
+- Symbols in all `libyabridge.so` and all Winelib `.so` files are now hidden by
+  default.
 
 ## [2.1.0] - 2020-11-20