win32: strncmp -> git__strncmp

The win32 C library is compiled cdecl, however when configured with
`STDCALL=ON`, our functions (and function pointers) will use the stdcall
calling convention.  You cannot set a `__stdcall` function pointer to a
`__cdecl` function, so it's easier to just use our `git__strncmp`
instead of sorting that mess out.

src/ignore.c

@@ -59,7 +59,7 @@ static int does_negate_pattern(git_attr_fnmatch *rule, git_attr_fnmatch *neg)
 	if (neg->flags & GIT_ATTR_FNMATCH_ICASE)
 		cmp = git__strncasecmp;
 	else
-		cmp = strncmp;
+		cmp = git__strncmp;
 
 	/* If lengths match we need to have an exact match */
 	if (rule->length == neg->length) {