setup.c: Fix some "symbol not declared" sparse warnings

setup.c: Fix some "symbol not declared" sparse warnings

In particular, sparse issues the "symbol 'a_symbol' was not declared.
Should it be static?" warnings for the following symbols:

    setup.c:159:3: 'pathspec_magic'
    setup.c:176:12: 'prefix_pathspec'

These symbols only require file scope, so we add the static modifier
to their declarations.

Signed-off-by: Ramsay Jones <redacted>
Signed-off-by: Junio C Hamano <redacted>