lavu/tests: move timer.h include earlier

In the next commit, timer.h will require a _GNU_SOURCE to be set before
including system headers. This commit prevents compilation failures.

libavutil/tests/softfloat.c

@@ -18,6 +18,8 @@
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  */
 
+#include "libavutil/timer.h"
+
 #include <inttypes.h>
 
 #include "libavutil/softfloat.h"