Added M_PI_2 to mavlink_conversions.h if not defined.

Added M_PI_2 definition to mavlink_conversions.h if not defined due to older math.h libraries not defining M_PI_2. I ran into this issue when compiling with the xc16 compiler/libraries for the pic16 microcontroller, which does not define M_PI or M_PI_2 anywhere.

asin(1) should be calculated at compile time, but if not it can be replaced with the actual value given enough precision.