gio: Fix various implicit conversions from size_t to smaller types

Basically various trivial instances of the following MSVC compiler
warning:
```
../gio/gio-tool-set.c(50): warning C4267: '=': conversion from 'size_t' to 'int', possible loss of data
```

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

gio/gdummyfile.c

@@ -239,7 +239,7 @@ static const char *
 match_prefix (const char *path, 
               const char *prefix)
 {
-  int prefix_len;
+  size_t prefix_len;
 
   prefix_len = strlen (prefix);
   if (strncmp (path, prefix, prefix_len) != 0)