WASAPI: use `check_include_file` instead of 'find_path` in CMake
WASAPI: use `check_include_file` instead of 'find_path` in CMake

# Current issue:

When attempting to compile under MSVC (#49), CMake can't find the WASAPI headers. I'm running Windows 10, Visual Studio Community 2015, and the latest SDK.

# Proposed fix:

Use `check_include_file` instead of `find_path`, which seems more semantically correct anyways.
andrewrk/libsoundio,andrewrk/libsoundio