Separate sndfile.hh implementation into C++ source

This is required to be able to include
"audiographer/sndfile/sndfile_writer.h" in more than
one source file. which would otherwise lead to
duplicate symbol: AudioGrapher::SndfileHandle::..

libs/audiographer/wscript

@@ -59,6 +59,7 @@ def build(bld):
     audiographer_sources = [
         'private/gdither/gdither.cc',
         'private/limiter/limiter.cc',
+        'src/general/sndfile.cc',
         'src/general/sample_format_converter.cc',
         'src/routines.cc',
         'src/debug_utils.cc',