Include `ORC_BEGIN_DECLS` and `ORC_END_DECLS` in `orc.h`
Ensures compatibility with C++.
This resolves linking issue in libvips that was introduced by commit bdf5269b (v0.4.39).
vector.cpp:(.text.vips__vector_init+0x6): undefined reference to `orc_init()'
collect2: error: ld returned 1 exit status