libavutil/mastering_display_metadata.h: change fields to be rationals as this...

libavutil/mastering_display_metadata.h: change fields to be rationals as this is how they are typically coded.

(this structure is not referenced anywhere yet)

Signed-off-by: Neil Birkbeck <neil.birkbeck@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>