Update for libdivecomputer pkg-config include file changes

Update for libdivecomputer pkg-config include file changes

Subsurface doesn't compile on OS X any more, because libdivecomputer
changed the way the header inclusion works: the include path from
pkg-config no longer includes the final "libdivecomputer" component, and
instead of doing

  #include <header.h>

for libdivecomputer headers, we're now supposed to do

  #include <libdivecomputer/header.h>

instead. Which is cleaner anyway.

The reason this only bit us on OS X is that I never trusted pkg-config
that much for non-system libraries on Linux (maybe it works, maybe it
doesn't, I've seen it go both ways), so on Linux we just used our own
version of the include path, and thus weren't affected by the
libdivecomputer config change.

Clean up the includes while at it - we no longer need (or want) the
device-specific header files, since we just use the generic functions.

Reported-by: Grischa Toedt <toedt@embl.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>