Add #include guards to statistics.h and the extern "C" for C++

If the extern "C" is missing, the C++ compiler will try to find a
function by its mangled name. Since the function is in a .c file,
there will be no mangled name.

Signed-off-by: Thiago Macieira <thiago@macieira.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Enable the code that was #if 0'ed out

Linking error is fixed.

Signed-off-by: Thiago Macieira <thiago@macieira.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>