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: NPaolo Bonzini <pbonzini@redhat.com>

configure

@@ -3923,7 +3923,7 @@ fi
 # check if memfd is supported
 memfd=no
 cat > $TMPC << EOF
-#include <sys/memfd.h>
+#include <sys/mman.h>
 
 int main(void)
 {

util/memfd.c

@@ -31,9 +31,7 @@
 
 #include "qemu/memfd.h"
 
-#ifdef CONFIG_MEMFD
-#include <sys/memfd.h>
-#elif defined CONFIG_LINUX
+#if defined CONFIG_LINUX && !defined CONFIG_MEMFD
 #include <sys/syscall.h>
 #include <asm/unistd.h>