WASAPI: Remove useless statement in CMake module
WASAPI: Remove useless statement in CMake module

AUDIOCLIENT_H, as declared here, is not an external cached variable, so calling mark_as_advanced() has no apparent effect (does not enable user to modify value, even in advanced mode). Even if it were cached, its value (success/failure at finding audioclient.h) should not be edited by the user.

Also modified whitespace to be more consistent with that found in CMakeLists.txt.
andrewrk/libsoundio,andrewrk/libsoundio