acinclude: Fix false positive search for prandom_u32

acinclude: Fix false positive search for prandom_u32

Searching random.h for prandom_u32 will also match when prandom_u32_max
is present and cause a false positive HAVE_PRANDOM_U32.  Fix this up
by looking for the parenthesis following prandom_u32 so it won't
match on prandom_u32_max.

Passes Travis:
https://travis-ci.org/gvrose8192/ovs-experimental/builds/595171808

Reviewed-by: Yifeng Sun <pkusunyifeng@gmail.com>
Signed-off-by: Greg Rose <gvrose8192@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>