Avoid using type 'gasmix_t': use 'struct gasmix' instead

Avoid using type 'gasmix_t': use 'struct gasmix' instead

libdivecomputer already uses 'gasmix_t' for its own gasmix thing.  I
don't like th eway we step on each others name spaces, but hey, might as
well just use 'struct gasmix' and avoid the typedef.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>