alignment: set alignment on structs

This uses an alternate idea for setting the structure alignments. Lets
see if this silents the ARM builder cast alignment issues.

GSlice (which is what is used for GObjects) guarantees a 2-pointer size
allocation, which means we can be ensured that the alignment is at least
8 bytes.

lib/sp-visualizer-ticks.c

@@ -34,7 +34,7 @@ struct _SpVisualizerTicks
   gint64 epoch;
   gint64 begin_time;
   gint64 end_time;
-} __attribute__((aligned(8)));
+};
 
 enum {
   TICK_MINUTES,