memfd: fix configure test

Recent glibc added memfd_create in sys/mman.h.  This conflicts with
the definition in util/memfd.c:

    /builddir/build/BUILD/qemu-2.11.0-rc1/util/memfd.c:40:12: error: static declaration of memfd_create follows non-static declaration

Fix the configure test, and remove the sys/memfd.h inclusion since the
file actually does not exist---it is a typo in the memfd_create(2) man
page.

Cc: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>