gve: Implement gettimex64 with -EOPNOTSUPP

In the Linux kernel, the following vulnerability has been resolved: gve: Implement gettimex64 with -EOPNOTSUPP gve implemented a ptp_clock for sole use of do_aux_work at this time. ptp_clock_gettime() and ptp_sys_offset() assume every ptp_clock has implemented either gettimex64 or gettime64. Stub gettimex64 and return -EOPNOTSUPP to prevent NULL dereferencing.