config.c: mark file-local function static

Commit 7192777d 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: NRamsay Jones <ramsay@ramsay1.demon.co.uk>
Explained-by: NJeff King <peff@peff.net>
Signed-off-by: NJonathan Nieder <jrnieder@gmail.com>

config.c

@@ -498,7 +498,7 @@ static int git_parse_signed(const char *value, intmax_t *ret, intmax_t max)
 	return 0;
 }
 
-int git_parse_unsigned(const char *value, uintmax_t *ret, uintmax_t max)
+static int git_parse_unsigned(const char *value, uintmax_t *ret, uintmax_t max)
 {
 	if (value && *value) {
 		char *end;