AP_Math: fix eulers example for PX4

libraries/AP_Math/examples/eulers/eulers.cpp

@@ -234,8 +234,12 @@ void test_frame_transforms(void)
 // generate a random float between -1 and 1
 static float rand_num(void)
 {
-    float ret = random();
-    return 2.0f * ret / RAND_MAX - 1.0f;
+#if CONFIG_HAL_BOARD == HAL_BOARD_PX4
+    /* random() isn't implemented for PX4 */
+    return 2.0f * rand() / MAX_RAND - 1.0f;
+#else
+    return 2.0f * random() / RAND_MAX - 1.0f;
+#endif
 }
 
 void test_matrix_rotate(void)