Ensure Subsurface still compiles with libdivecomputer 0.4.x

The OSTC3 functions we need were introduced in 0.5.0 (more specifically,
in commit 56e2dec7266e71e725604a4a8bff5e61f3d62259).

Signed-off-by: Thiago Macieira <thiago@macieira.org>