config.c: mark file-local function static

config.c: mark file-local function static

Commit 7192777 refactors git_parse_ulong, which is public, into a more
generic function.  But since we kept the git_parse_ulong wrapper, only
that part needs to be public; nobody outside the file calls the
lower-level git_parse_unsigned.

Noticed with sparse.  ("'git_parse_unsigned' was not declared. Should
it be static?")

Signed-off-by: Ramsay Jones <redacted>
Explained-by: Jeff King <redacted>
Signed-off-by: Jonathan Nieder <redacted>