The definition and declaration of gr_sync_interpolator contains a typo.

1 63       int fixed_rate_ninput_to_noutout(int ninput);
2 64       int fixed_rate_noutput_to_ninput(int noutput);

The correct function name is fixed_rate_ninput_to_nout*p*ut. This a virtual function defined first for gr_block. The error is in both files (header + source). Since definition and declaration are identical and there is a default implementation in gr_sync_block, no compiler error occurs.