Define FLUIDSYNTH_API on OS/2
Define FLUIDSYNTH_API on OS/2

Previously, CMake on OS/2 exported all the symbols unconditionally. Now
it exports necessary symbols only. As a result, it's necessary to
define FLUIDSYNTH_API correctly.

Addresses #678

FluidSynth/fluidsynth,FluidSynth/fluidsynth,FluidSynth/fluidsynth,FluidSynth/fluidsynth