remove all <cmath> usage

* the NuttX c++ library is incomplete, let's avoid including it until we have a real standard library in place

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

@@ -35,7 +35,7 @@
 
 #include <px4_platform_common/defines.h>
 
-#include <cmath>
+#include <math.h>
 
 namespace math
 {

src/lib/mathlib/math/filter/LowPassFilter2pVector3f.cpp

@@ -35,7 +35,7 @@
 
 #include <px4_platform_common/defines.h>
 
-#include <cmath>
+#include <math.h>
 
 namespace math
 {