fuzz: add missing header for rcu_enable_atfork

In 45222b9a, I fixed a broken check for rcu_enable_atfork introduced
in d6919e4c. I added a call to rcu_enable_atfork after the
call to qemu_init in fuzz.c, but forgot to include the corresponding
header, breaking --enable-fuzzing --enable-werror builds.

Fixes: 45222b9a ("fuzz: fix broken qtest check at rcu_disable_atfork")
Signed-off-by: Alexander Bulekov <alxndr@bu.edu>
Message-Id: <20200708200104.21978-3-alxndr@bu.edu>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

tests/qtest/fuzz/fuzz.c

@@ -19,6 +19,7 @@
 #include "sysemu/runstate.h"
 #include "sysemu/sysemu.h"
 #include "qemu/main-loop.h"
+#include "qemu/rcu.h"
 #include "tests/qtest/libqtest.h"
 #include "tests/qtest/libqos/qgraph.h"
 #include "fuzz.h"