timer test: use 'volatile' for locals

GCC seems to be failing to follow the letter of the C spec by allowing extra
precision in floating point values to persist across assignments which are
optimised away.

Force its hand by using 'volatile' on the locals in question.

Bug: https://gitlab.gnome.org/GNOME/glib/issues/820
Forwarded: yes

glib/tests/timer.c

@@ -30,7 +30,7 @@ static void
 test_timer_basic (void)
 {
   GTimer *timer;
-  gdouble elapsed;
+  volatile gdouble elapsed;
   gulong micros;
 
   timer = g_timer_new ();
@@ -65,7 +65,7 @@ static void
 test_timer_stop (void)
 {
   GTimer *timer;
-  gdouble elapsed, elapsed2;
+  volatile gdouble elapsed, elapsed2;
 
   timer = g_timer_new ();