Make sure malloc is linked into gdb.cp/oranking.cc.

Make sure malloc is linked into gdb.cp/oranking.cc.

On some platforms, e.g., arm-eabi-none, we need to make certain that
malloc is linked into the program because the test suite uses function
calls requiring malloc:

(gdb) p foo101("abc")
evaluation of this expression requires the program to have a function "malloc".

gdb/testsuite/ChangeLog

	* gdb.cp/oranking.cc (dummy): New function to grab malloc.
	(main): Call it.