SITL: fixed some build warnings

libraries/SITL/SIM_Aircraft.cpp

@@ -49,7 +49,6 @@ Aircraft::Aircraft(const char *home_str, const char *frame_str) :
     gyro_noise(radians(0.1f)),
     accel_noise(0.3),
     rate_hz(400),
-    last_time_us(0),
     autotest_dir(NULL),
 #ifdef __CYGWIN__
     min_sleep_time(20000)

libraries/SITL/SIM_Aircraft.h

@@ -130,8 +130,8 @@ protected:
     double rand_normal(double mean, double stddev);
 
 private:
-    uint64_t last_time_us;
-    uint32_t frame_counter;
+    uint64_t last_time_us = 0;
+    uint32_t frame_counter = 0;
     const uint32_t min_sleep_time;
 };