AP_Stats: correct compilation when AP_AHRS is not enabled

[peterbarker]

CarbonixF405 peripheral uses this library but not AP_AHRS

/home/pbarker/gcc/gcc-arm-none-eabi-10-2020-q4-major/bin/../lib/gcc/arm-none-eabi/10.2.1/../../../../arm-none-eabi/bin/ld: lib/libAP_Periph_libs.a(AP_Stats.cpp.0.o): in function `AP_Stats::update_distance_flown()':
AP_Stats.cpp:(.text._ZN8AP_Stats21update_distance_flownEv+0x28): undefined reference to `AP::ahrs()'
/home/pbarker/gcc/gcc-arm-none-eabi-10-2020-q4-major/bin/../lib/gcc/arm-none-eabi/10.2.1/../../../../arm-none-eabi/bin/ld: AP_Stats.cpp:(.text._ZN8AP_Stats21update_distance_flownEv+0x2e): undefined reference to `AP_AHRS::healthy() const'
/home/pbarker/gcc/gcc-arm-none-eabi-10-2020-q4-major/bin/../lib/gcc/arm-none-eabi/10.2.1/../../../../arm-none-eabi/bin/ld: AP_Stats.cpp:(.text._ZN8AP_Stats21update_distance_flownEv+0x42): undefined reference to `AP_AHRS::get_location(Location&) const'
collect2: error: ld returned 1 exit status