So, this looks like an 'external declaration' of a function that belongs to another file.
But I can see it is not, as the function compoundValueBson comes right after.
This line is completely useless.
What you need to do is to move it to the header file (MongoCommonUpdate.h) and don't forget the (useless) keyword 'extern':

    extern void compoundValueBson(...);

Now, files that want to use this function simply includes MongoCommonUpdate.h