Work around the following

    (gdb) Python Exception <class 'gdb.error'> 'fclose@@GLIBC_2.2.5' has
    unknown return type; cast the call to its declared return type:
    (gdb) Error occurred in Python: 'fclose@@GLIBC_2.2.5' has unknown
    return type; cast the call to its declared return type

This is due to GDB not being able to pick up and use the return types from
debug info for external declarations.

2019-07-14  Vladislav Ivanishin <vlad@ispras.ru>

        * gdbhooks.py (DumpFn.invoke): Add explicit casts of return values of
        fopen and fclose to their respective types.
        (DotFn.invoke): Ditto.

From-SVN: r273480