Fix warnings redundant redeclaration of jl_n_gcthreads

[fingolfin]

Specifically, this warning I see in tests against Julia nightly e.g. in this log:

/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

[lgoettgens]

@giordano could you please have a look at this (or ping someone you deem more suitable for a review)? Thanks!

[giordano]

Looks reasonable to me, maybe @gbaraldi can have a quick look (it's a one line change)?