AP_NavEKFx: apply -O2 to AP_NavEKFx.h

[peterbarker]

the core.h file adjusts the optimisation level; use it when including the AP_NavEKFx.h header file

this should be done consistently across the .cpp files to ensure everything is compiled with the same optimisation level.

Board                    AP_Periph  blimp  bootloader  copter  heli  iofirmware  plane  rover  sub
CubeOrange-periph-heavy  *                                                                     
Durandal                            -80    *           -72     -72               -72    -80    -80
Hitec-Airspeed           *                                                                     
KakuteH7-bdshot                     -96    *           -88     -88               -88    -88    -88
MatekF405                           -72    *           -64     -64               -64    -64    -64
Pixhawk1-1M-bdshot                  -64                -64     -56               -56    -56    -56
f103-QiotekPeriph        *                                                                     
f303-Universal           *                                                                     
iomcu                                                                *                         
revo-mini                           -72    *           -64     -64               -64    -64    -64
skyviper-v2450                                         -64