[gdb/contrib] Fix "sofar->so far" misspelling

I forgot to follow up on a review comment and fix the "sofar->so far"
misspelling [1].

Fix this by adding it to gdb/contrib/common-misspellings.txt.

Tested on x86_64-linux.

[1] https://sourceware.org/pipermail/gdb-patches/2024-September/211894.html