acinclude: Fix build with kernels with prandom* moved to prandom.h.

acinclude: Fix build with kernels with prandom* moved to prandom.h.

Recent commit c0842fbc1b18 ("random32: move the pseudo-random 32-bit
definitions to prandom.h") in upstream kernel moved the definition
of prandom_* functions from random.h to prandom.h.  This change was
also backported to stable kernels.

Fixing our configure script to look for these functions in a new
location and avoid build failures:

  datapath/linux/compat/include/linux/random.h:11:19:
    error: redefinition of 'prandom_u32_max'

Acked-by: Greg Rose <gvrose8192@gmail.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>