Use size_t in gdb_bfd_section_data

BFD recently changed bfd_mmap to use size_t, not bfd_size_type.  This
patch updates gdb_bfd_section_data to follow.  Without this patch, if
the two types ever differ, gdb would fail to build.

Approved-By: Simon Marchi <simon.marchi@efficios.com>

gdb/gdb_bfd.c

@@ -71,9 +71,9 @@ gdb_bfd_unlock (void *ignore)
 struct gdb_bfd_section_data
 {
   /* Size of the data.  */
-  bfd_size_type size;
+  size_t size;
   /* If the data was mmapped, this is the length of the map.  */
-  bfd_size_type map_len;
+  size_t map_len;
   /* The data.  If NULL, the section data has not been read.  */
   void *data;
   /* If the data was mmapped, this is the map address.  */