Fix public issue #44: Make the definition and declaration of CompressFragment
identical, even regarding cv-qualifiers.

This is required to work around a bug in the Solaris Studio C++ compiler
(it does not properly disregard cv-qualifiers when doing name mangling).

R=sanjay


git-svn-id: http://snappy.googlecode.com/svn/trunk@44 03e5f5b5-db94-4691-08a0-1a8bf15f6143