package/libjson: fix build with gcc 11

package/libjson: fix build with gcc 11

GCC 11 defaults to C++17. Fix the following build failure with gcc 11:

In file included from _internal/Source/JSONDefs.h:12,
                 from _internal/Source/JSONDebug.h:4,
                 from _internal/Source/JSONNode.h:4,
                 from _internal/Source/JSONNode.cpp:1:
_internal/Source/JSONDefs/GNU_C.h:58:28: error: ISO C++17 does not allow dynamic exception specifications
   58 |     #define json_throws(x) throw(x)
      |                            ^~~~~

Fixes:
 - http://autobuild.buildroot.org/results/1e66dff705bbb38e7e0f0e5864ce794b4345dcc6

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>