Close file handle on success in pc_opensrc.

`pc_opensrc` is leaking file handles after the `src_file_t` refactor.

I'm fairly certain that the file handle shouldn't be sticking around anyway, as the `fp` field of the `src_file_t` states:
```// Set if writing.```
and the field isn't set within `pc_opensrc`, so it simply goes unused and leaks.

This fixes an issue I encountered when dealing with a plugin with many included files (100+).