If -fvisibility=hidden isn't supported, try -xldscope=hidden.

That's the option for newer versions of Sun^WOracle C.

Change-Id: I62c12d5870d84587f81a8789732675021523e9ed
Reviewed-on: https://code.wireshark.org/review/2769
Reviewed-by: Guy Harris <guy@alum.mit.edu>