power: Fix arithmetic with the idle-dim multiplier

Spotted by Giovanni Campagna.

https://bugzilla.gnome.org/show_bug.cgi?id=692192

plugins/power/gsd-power-constants.h

@@ -26,7 +26,7 @@
 #define IDLE_DIM_BLANK_DISABLED_MIN                     60 /* seconds */
 
 /* Which fraction of the idle-delay is the idle-dim delay */
-#define IDLE_DELAY_TO_IDLE_DIM_MULTIPLIER                4/5
+#define IDLE_DELAY_TO_IDLE_DIM_MULTIPLIER                4.0/5.0
 
 /* The dim delay under which we do not bother dimming */
 #define MINIMUM_IDLE_DIM_DELAY                          10 /* seconds */