intfloat: Don't use designated initializers in the public headers

intfloat.h is a public header, and is now (since a1245d5c) included
by mathematics.h, which many external callers include.

This fixes building third party applications that include
mathematics.h in a language that doesn't support designated
initalizers.

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