Add missing #ifdef __cplusplus in dat.h

[BLumia]

Find this issue when using thtk inside a c++ project. This PR simply add the missing code in dat.h .

Refer to thtk/thtk/dat.h(L145) , it seems we only have a single unpaired } if we are using c++.

What added:

#ifdef __cplusplus
extern "C" {
#endif

Thanks
BLumia