w32: Add another pinentry search path

* common/homedir.c (get_default_pinentry_name): Try ../bin/pinentry.exe

In an installation layout where GnuPG is installed as a component
of another software it might be useful to have pinentry placed
in a different bin directory without the forced folder name of
Gpg4win.

(cherry picked from commit b4cb91d5fbe2b8917d76d12eb72aaac0d97ed596)