Changes DLL_EXPORT to SODIUM_DLL_EXPORT.

This macro conflicts with other projects.  This results in the inability to
build one DLL that depends on libsodium if the other DLL also uses the
DLL_EXPORT macro to control visibility of library symbols.  Since the choice of
name for this macro is arbitrary, use of a library prefix is preferred.
Merge pull request #197 from AndreLouisCaron/vs2008

Fixes VS 2008 build.