Remove an explicit include of debugapi.h

The documentation for IsDebuggerPresent says that one just should
include windows.h, as that one is an umbrella header that includes
the header that declares IsDebuggerPresent. In older Windows SDKs,
debugapi.h didn't exist and IsDebuggerPresent was declared in
winbase.h (also included by windows.h).

This should fix issue #2822 properly.