Fix warnings redundant redeclaration of `jl_n_gcthreads` (#59630)
Specifically, this warning I see in tests against Julia nightly e.g. [in
this
log](https://github.com/gap-system/gap/actions/runs/17926134820/job/50972543899?pr=6122#step:7:234):
```
/opt/hostedtoolcache/julia/nightly/x64/include/julia/julia.h:2058:25: error: redundant redeclaration of ‘jl_n_gcthreads’ [-Werror=redundant-decls]
2058 | extern JL_DLLIMPORT int jl_n_gcthreads;
| ^~~~~~~~~~~~~~
/opt/hostedtoolcache/julia/nightly/x64/include/julia/jl_exported_data.inc:159:8: note: previous declaration of ‘jl_n_gcthreads’ with type ‘int’
159 | ZZ(jl_n_gcthreads, int) \
| ^~~~~~~~~~~~~~
/opt/hostedtoolcache/julia/nightly/x64/include/julia/julia.h:1043:49: note: in definition of macro ‘XX’
1043 | #define XX(name, type) extern JL_DLLIMPORT type name;
| ^~~~
/opt/hostedtoolcache/julia/nightly/x64/include/julia/julia.h:1044:1: note: in expansion of macro ‘JL_EXPORTED_DATA_SYMBOLS’
1044 | JL_EXPORTED_DATA_SYMBOLS(XX)
| ^~~~~~~~~~~~~~~~~~~~~~~~
```
The code seems to be the same in Julia 1.10, 1.11, 1.12 -- but I only
see the warning in builds against nightly. I have no idea why...
CC @lgoettgens
