issue-613: remove friend declaration from HeapLeakChecker

This applies patch by davide.italiano@10gen.com:

heap-checker.h contains the following friend declaration of main:
friend int main(int, char**).

C99 allows another declaration of main, i.e. int main(int, char**,
char**), and if code uses it and includes the heap-checker header,
this might result in a conflict, e.g.

error: declaration of C function 'int main(int, char**, char**)' conflicts with
 int main(int argc, char* argv[], char** envp)

Actually the comment above the friend declaration of main() mentions
that this is required to get the unittest working and for other
internal usage, but I'm not completely sure if this is true as long as
I'm able to build and run the unittest removing the declaration.