Added 'extern "C" { ... }' to the header files to force C linkage.

I assume the only files users should directly be including are:
 * ottery.h
 * ottery_nolock.h
 * ottery_st.h

ottery_version.h does not have function definitions so linkage doesn't matter, and ottery_common.h is covered because linkage is specified before it is included from the 3 header files.