"%" PRIi64 is reported as non-constant format string

    int64_t x = 123;
    sprintf(buffer, "%" PRIi64, x);

produces the following warning

> Check to be sure that the non-constant format string passed as argument 2 to this function call does not come from an untrusted source that could have added formatting characters that the code is not prepared to handle.

_PRIi64_ is defined in standard header <inttypes.h> as a constant string.
