r600: silence the signed overflow warning like radeonsi

r600: silence the signed overflow warning like radeonsi

r600_gpu_load.c: In function ‘r600_gpu_load_thread’:
../../../../src/util/os_time.h:82:7: warning: assuming signed overflow does not occur when assuming that (X + c) >= X is always true [-Wstrict-overflow]
    if (start <= end)