package/sconeserver: fix build with gcc 11

package/sconeserver: fix build with gcc 11

Fix the following build failure with gcc 11:

In file included from ../sconex/sconex.h:229,
                 from ../sconex/Descriptor.h:63,
                 from Descriptor.cpp:22:
Descriptor.cpp: In member function 'void scx::Descriptor::add_stream(scx::Stream*)':
Descriptor.cpp:150:22: error: ordered comparison of pointer with integer zero ('scx::Stream*' and 'int')
  150 |   DEBUG_ASSERT(stream>=0,"add_stream() Invalid stream");
      |                ~~~~~~^~~
Descriptor.cpp: In member function 'bool scx::Descriptor::remove_stream(scx::Stream*)':
Descriptor.cpp:204:22: error: ordered comparison of pointer with integer zero ('scx::Stream*' and 'int')
  204 |   DEBUG_ASSERT(stream>=0,"remove_stream() Invalid stream");
      |                ~~~~~~^~~

Fixes:
 - http://autobuild.buildroot.org/results/ccc9562e83fd2bd312d21b3124be42dfe4b7e850

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>