Set -fvisibility=hidden for macos/static builds.

Author: pmienk

.github/workflows/ci.yml

@@ -97,7 +97,7 @@ jobs:
             mbedtls: ""
             zmq: "--build-zmq"
             cc: "clang"
-            flags: "-Os -fPIE"
+            flags: "-Os -fvisibility=hidden -fPIE"
             packager: "brew"
             packages: ""