gdb-add-index.sh: Remove use of non posix 'local'

gdb-add-index.sh: Remove use of non posix 'local'

While working on gdb-add-index.sh, it appeared that it uses the non
POSIX 'local' keyword.  Instead of using local to allow variable
shadowing, I rename the local one to avoid name conflicts altogether.

This commit gets rid of the following shellcheck warning:

    In gdb-add-index.sh line 63:
        local file="$1"
        ^--------^ SC2039: In POSIX sh, 'local' is undefined.

gdb/ChangeLog:

	* contrib/gdb-add-index.sh: Avoid variable shadowing and get
	rid of 'local'.