build: fix FreeBSD build

build: fix FreeBSD build

Commit 7c9a2d88 cleaned up too many headers; FreeBSD builds
failed due to:

util/virutil.c:556: warning: implicit declaration of function 'canonicalize_file_name'

(Not sure which Linux header leaked this declaration, but gnulib
only guarantees it in stdlib.h)

libvirt.c:956: warning: implicit declaration of function 'virGetUserConfigDirectory'

(Here, a build on Linux was picking up virutil.h indirectly via
one of the conditional driver headers, where that driver was not
being built on my FreeBSD setup)

* src/util/virutil.c (includes): Need <stdlib.h> for
canonicalize_file_name.
* src/libvirt.c (includes): Use "virutil.h" unconditionally,
rather than relying on conditional indirect inclusion.

Signed-off-by: Eric Blake <eblake@redhat.com>