Added accelerometer simulator

The simulator satisfies the dependencies for an accelerometer
being present.

The accel code compiles but is not fully functional.

Signed-off-by: Mark Charlebois <charlebm@gmail.com>

src/lib/mathlib/math/filter/LowPassFilter2p.cpp

@@ -37,6 +37,7 @@
 /// @brief	A class to implement a second order low pass filter 
 /// Author: Leonard Hall <LeonardTHall@gmail.com>
 
+#include <px4_defines.h>
 #include "LowPassFilter2p.hpp"
 #include "math.h"