sys/types32.h: Remove struct timeval32 from libspl's header (#16491)

macOS Sequoia's sys/sockio.h, as included by various bootstrap tools
whilst building FreeBSD, has started to include net/if.h, which then
includes sys/_types/_timeval32.h and provide a conflicting definition
for struct timeval32. Since this type is entirely unused within OpenZFS,
simply delete the type rather than adding in some kind of OS detection.

This fixes building FreeBSD on macOS Sequoia (Beta).

Signed-off-by: Jessica Clarke <jrtc27@jrtc27.com>
Reviewed-by: Rob Norris <rob.norris@klarasystems.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>