build: Enabled DEBUGBUILD in Visual Studio debug builds

Defined the DEBUGBUILD pre-processor variable to allow extra logging,
which is particularly useful in debug builds, as we use this and Visual
Studio typically uses _DEBUG.

We could define DEBUBBUILD, in curl_setup.h, when _MSC_VER and _DEBUG is
defined but that would also affect the makefile based builds which we
probably don't want to do.