As rbalint points out, this was an exported fn. It also is
the only way for a libsubid user to do what it does, so let's
not drop it.
This reverts commit 477c8e6f42.
C++ requires extern "C" linkage specification to call functions from a C
library. Enclose the function definitions in subid.h in an extern "C"
block if compiling in C++ mode to achieve this.
Signed-off-by: Alois Wohlschlager <alois1@gmx-topmail.de>
