dshowsrcwrapper: Update build instructions. Add _builddir to include search path.
When building dshowsrcwrapper with a recent (2017) Visual and SDK I encountered an error that does definitely not speak for itself (cannot include ctype.h: no such file or directory). Update BUILD.txt to mention the error and the fix. Additionally, add _builddir to the include search path so that config.h is found when building from the Cerbero build tree.
Edited by Jérôme Laheurte