For Sun C with linker scoping specifiers, define WS_DLL_PUBLIC_DEF.

This may let us, at least for those compilers, make the default "hidden"
with -xldscope=hidden.

Change-Id: I94e10733c2aba0ff8d77a8bf4f1dc8ecc8b2a47f
Reviewed-on: https://code.wireshark.org/review/2766
Reviewed-by: Guy Harris <guy@alum.mit.edu>