gmodule: 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>

gmodule/gmodule-win32.c

@@ -205,7 +205,7 @@ static gchar*
 _g_module_build_path (const gchar *directory,
 		      const gchar *module_name)
 {
-  gint k;
+  size_t k;
 
   k = strlen (module_name);