headers: Fix include order of fvec.h/ivec.h/dvec.h referring to each other

Previously, these headers expanded properly if dvec.h was included
first, but not if fvec.h or ivec.h were included first.

Move the cyclical includes last, so that the bits that these headers
are expected to declare are declared before the next level header
is included.

Signed-off-by: Martin Storsjö <martin@martin.st>