readelf: Pull advance_pc() in file scope

[Timm Bäder via Elfutils-devel]

Giving this another shot since pulling all the parsing code into
libdwP.h seems a bit overkill (and potentially buggy of course). The
downside is that run_advance_pc() takes 6 arguments instead of the 4
that it takes in libdwarf_getsrclines.c