Avoid exporting non-public symbols

Between libusbmuxd 1.0.8 and 1.0.9, some private symbols were removed from the library, which technically breaks ABI. This pull request adds -fvisibility=hidden to GLOBAL_CFLAGS and explicitly exports the public symbols as per [1] to avoid exporting these symbols as part of the ABI and avoiding future breakage.

This also effectively decouples the iproxy binary from libusbmuxd's private symbols so that they can be upgraded separately.

[1] http://gcc.gnu.org/wiki/Visibility