rstest: OpenMP: mark std::err as shared()
rstest: OpenMP: mark std::err as shared()

GCC complains, clang does not
https://bugs.llvm.org/show_bug.cgi?id=41768
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90366
https://godbolt.org/z/hC0cbC

LebedevRI/rawspeed,LebedevRI/rawspeed,darktable-org/rawspeed,darktable-org/rawspeed,LebedevRI/rawspeed,darktable-org/rawspeed,darktable-org/rawspeed,LebedevRI/rawspeed,darktable-org/rawspeed,LebedevRI/rawspeed