fix rdtsc usage

One user had a problem compiling oaisim.
This commit fixes it.

The compilation log was saying:

    targets/RT/USER/lte-ue.c:
    In function UE_thread_rxn_txnp4:

    openair2/UTIL/LOG/log.h:370:3:
    error: inconsistent operand constraints in an asm

         __asm__ volatile ("rdtsc" : "=a" (a), "=d" (d));

         ^