adjust missing NO_PACKING and MODPLUG_NO_FILESAVE ifdefs:

CSoundFile::PackSample() and CSoundFile::CanPackSample() are only
used when writing samples and only when NO_PACKING is not defined.
sample write, in turn, is only available when MODPLUG_NO_FILESAVE
is not defined. therefore, wrap them with MODPLUG_NO_PACKING and
NO_FILESAVE ifdefs in sndfile.h, and move the MODPLUG_NO_PACKING
ifdef in sndfile.cpp to cover PackSample() and CanPackSample() too.
This doesn't affect the C-only interface, however CSoundFile class
in the C++ interface is changed, therefore MODPLUG_LIBRARY_VERSION
in configure.ac would _probably_ need changing before a release.