gdb/options: fix copy&paste error in string_option_def

gdb/options: fix copy&paste error in string_option_def

Spotted what appears to be a copy&paste error in string_option_def,
the code for string handling writes the address fetching callback
function into the option_def::var_address::enumeration location,
rather than option_def::var_address::string.

Of course, this works just fine as option_def::var_address is a union,
and all of its members are function pointers, so they're going to be
the same size on every target GDB cares about.

But it doesn't hurt to be correct, so fixed in this commit.

There should be no user visible changes after this commit.