qt: osx: fix hidden symbol visibility

Fixes default hidden symbol visibility for our linux->osx cross build. Without
this change, the check for working -fvisibility=hidden fails, and all symbols
are visible by default.

Ugly as this is, it's just a simple find/replace to fix a bug in Qt's configure.
They assume in an "XPLATFORM_MAC" block that the builder is capable of running
osx programs. This should be "BUILD_ON_MAC" instead.