libzpool: Fix gettimeofday() include

Fixes the following build breakage:

    CC bsd/cddl/contrib/opensolaris/lib/libzpool/common/kernel.o
  ../../bsd/cddl/contrib/opensolaris/lib/libzpool/common/kernel.c: In function ‘cv_timedwait’:
  ../../bsd/cddl/contrib/opensolaris/lib/libzpool/common/kernel.c:322:2: error: implicit declaration of function ‘gettimeofday’ [-Werror=implicit-function-declaration]
    if (gettimeofday(&tv, NULL) != 0)
  ^
  cc1: all warnings being treated as errors

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>