performance tool: change unit from 'ns' to 'unit of time'
performance tool: change unit from 'ns' to 'unit of time'

Since the rohc_test_performance tool uses clock_gettime(), the measures are
not performed in seconds or nanoseconds anymore. The unit of time is arbitrary.

didier-barvaux/rohc,didier-barvaux/rohc,didier-barvaux/rohc,didier-barvaux/rohc,didier-barvaux/rohc