Add extern 'C' guard for c++ compilation

9 jobs for cpp_header in 5 minutes and 58 seconds (queued for 1 second)
Status Job ID Name Coverage
  Style
passed #85869
nix kvm
clang-format

00:00:05

passed #85868
nix kvm
repoquality

00:00:03

passed #85870
nix kvm
style:docs

00:00:40

 
  Build
passed #85875
cuda
make:chimera

00:00:58

passed #85871
nix kvm
make:generic

00:01:00

passed #85874
knl
make:knl

00:03:55

passed #85872
nix kvm
make:out-of-tree

00:01:12

passed #85873
nix kvm
make:valgrind

00:02:00

 
  Distribution
passed #85876
nix kvm
distcheck

00:01:19