stubs: avoid duplicate symbols in libqemuutil.a

stubs: avoid duplicate symbols in libqemuutil.a

qapi_event_send_device_deleted is always included (together with the
rest of QAPI) in libqemuutil.a if either system-mode emulation or tools
are being built, and in that case the stub causes a duplicate symbol
to appear in libqemuutil.a.

Add the symbol only if events are not being requested.

Cc: qemu-stable@nongnu.org
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit 388b849fb6c33882b481123568995a749a54f648)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>