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>

stubs/meson.build

@@ -55,7 +55,12 @@ endif
 if have_user
   # Symbols that are used by hw/core.
   stub_ss.add(files('cpu-synchronize-state.c'))
-  stub_ss.add(files('qdev.c'))
+
+  # Stubs for QAPI events.  Those can always be included in the build, but
+  # they are not built at all for --disable-system --disable-tools builds.
+  if not (have_system or have_tools)
+    stub_ss.add(files('qdev.c'))
+  endif
 endif
 
 if have_system