build: fix use of mmap

build: fix use of mmap

Commit bfe7721d introduced a regression, but only on platforms
like FreeBSD that lack posix_fallocate and where mmap serves as
a nice fallback for safezero.

util/virfile.c: In function 'safezero':
util/virfile.c:837: error: 'PROT_READ' undeclared (first use in this function)

* src/util/virutil.c (includes): Move use of <sys/mman.h>...
* src/util/virfile.c (includes): ...to the file that uses mmap.

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