gdb.python/py-nested-maps.exp C++ify

gdb.python/py-nested-maps.exp C++ify

This adjusts gdb.python/py-nested-maps.c to make it buildable as C++ program.

key_t is renamed because of:

  src/gdb/testsuite/gdb.python/py-nested-maps.c:23:8: error: definition of type 'key_t' conflicts with typedef of the same name
  struct key_t
	 ^
  /usr/include/x86_64-linux-gnu/sys/types.h:121:17: note: 'key_t' declared here
  typedef __key_t key_t;
		  ^

gdb/testsuite/ChangeLog:

	* gdb.python/py-nested-maps.c (struct key_t): Rename to...
	(struct my_key_t): ... this.  Adjust all references.
	(struct value_t): Rename to ...
	(struct my_value_t): ... this.  Adjust all references.
	(create_map, add_map_element, create_map_map)
	(add_map_map_element): Add casts.