Replace _bx_export_ with _public_

Modern projects, such as systemd, now use helpful macros like "_public_"
to make their purpose in code quite clear. Although we are indeed
exporting functionality, we're making that functionality public within
our API.

Note: The "public" keyword cannot be used as it is reserved in C++,
therefore to enable compatibility with C++ code, "_public_" was chosen.

Signed-off-by: Michael Ikey Doherty <michael.i.doherty@intel.com>