Use top_srcdir for path to opts.ggo

[ferdnyc]

This fixes out-of-tree builds, which otherwise will fail when attempting to run gengetopt:

$ mkdir build; cd build
$ ../configure
$ make
[...]
make[2]: Entering directory '.../mmv/build'
gengetopt < opts.ggo --unamed-opts
/bin/sh: line 1: opts.ggo: No such file or directory
make[2]: *** [Makefile:2205: cmdline.h] Error 1
make[2]: Leaving directory '.../mmv/build'
make[1]: *** [Makefile:1755: all-recursive] Error 1
make[1]: Leaving directory '.../mmv/build'
make: *** [Makefile:1556: all] Error 2
```

[rrthomas]

Many thanks for this!