girepository: 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>

girepository/girwriter.c

@@ -1338,7 +1338,8 @@ gi_ir_writer_write (GIRepository *repository,
                     gboolean    show_all)
 {
   FILE *ofile;
-  size_t i, j;
+  size_t i;
+  unsigned int j;
   char **dependencies;
   Xml *xml;