fix c++ pthread.h path for linux

c++ sodium was failing on linux with this error:

sodium/c++/sodium/transaction.h:21:29: fatal error: pthread/pthread.h: No such file or directory
compilation terminated.
make[2]: *** [CMakeFiles/libsodium.dir/sodium/transaction.cpp.o] Error 1
make[1]: *** [CMakeFiles/libsodium.dir/all] Error 2
make: *** [all] Error 2