AP_Math: narrow enumeration to uint8_t to avoid narrowing-warnings

/home/pbarker/rc/ardupilot/libraries/DataFlash/LogFile.cpp:361:25:
error: non-constant-expression cannot be narrowed from type 'enum
Rotation' to 'uint8_t' (aka 'unsigned char') in initializer list
[-Wc++11-narrowing]
        orient1       : s0 ? s0->orientation() : ROTATION_NONE,

libraries/AP_Math/rotations.h

@@ -24,7 +24,7 @@
 
 // NOTE!! these rotation values are stored to EEPROM, so be careful not to
 // change the numbering of any existing entry when adding a new entry.
-enum Rotation {
+enum Rotation : uint8_t {
     ROTATION_NONE                = 0,
     ROTATION_YAW_45              = 1,
     ROTATION_YAW_90              = 2,