gc-debug: fix time unit in jl_print_gc_stats

[inkydragon]

jl_hrtime() return nanosec. Then ptime is also in nanosec.
Need to convert unit to seconds with the jl_ns2s function.

julia/src/sys.c

Lines 461 to 462 in 7fc8646

	// Returns time in nanosec
	JL_DLLEXPORT uint64_t jl_hrtime(void) JL_NOTSAFEPOINT

julia/src/gc-debug.c

Line 1007 in 7fc8646

process_t0 = jl_hrtime();

NOTE: those codes need flag: CFLAGS = -DGC_FINAL_STATS