[gdb/testsuite] Fix gdb.base/condbreak-multi-context.exp with gcc 4.8.5

With gcc 4.8.5, I run into:
...
Running gdb.base/condbreak-multi-context.exp ...
gdb compile failed, condbreak-multi-context.cc:21:11: warning: non-static \
  data member initializers only available with -std=c++11 or -std=gnu++11 \
  [enabled by default]
   int b = 20;
           ^
...

Fix this by making it a static const.

Tested on x86_64-linux, with gcc 4.8.5, 7.5.0 and clang 13.0.1.