build: fix file mode of shell scripts (644 -> 755)

Some shell scripts are not executable, so fix their file modes:

    $ git grep -Elz '^#!/bin/(ba)?sh' | xargs -0 -I '{}' \
      chmod +x '{}'

Files changed:

* src/fgit/fgit-uninstall.sh
* src/tools/extract_errnos.sh

Note: I have manually checked that the files above are indeed intended
to be executable directly and not just sourced, as a script of the
latter kind could also contain a shebang (for example, to help ensure
proper syntax highlighting).

Misc: The affected files were added on commit e46dd3e ("git-install",
2017-02-04) and on commit 081d1fb ("Add seccomp errno filter support",
2015-09-23) / PR #66, respectively.