EKF: improve compatibility with ARM cross compiler
EKF: improve compatibility with ARM cross compiler

This change removes the following compiler error when building using the ARM cross compiler.

/Users/paul/src/Firmware/src/lib/ecl/EKF/ekf_helper.cpp:45:12: error: 'std::abs' has not been declared
 using std::abs;

PX4/ecl,PX4/ecl,PX4/ecl,PX4/ecl,PX4/ecl