Update cpu.lua (#506)

I noticed once that the CPU usage was being reported as being '-1%', I dug around the code for a while and think that the change at line 52 should make sure no negative values are obtained.

Co-authored-by: Pratik Devkota <[email protected]>

widget/cpu.lua

@@ -49,7 +49,7 @@ local function factory(args)
                 -- Read current data and calculate relative values.
                 local dactive = active - core.last_active
                 local dtotal  = total - core.last_total
-                local usage   = math.ceil((dactive / dtotal) * 100)
+                local usage   = math.ceil(math.abs((dactive / dtotal) * 100))
 
                 core.last_active = active
                 core.last_total  = total