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.

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2017-05-03  Keith Seitz  <keiths@redhat.com>
+
+	* gdb.cp/oranking.cc (dummy): New function to grab malloc.
+	(main): Call it.
+
 2017-04-27  Keith Seitz  <keiths@redhat.com>
 
 	* gdb.cp/oranking.cc (test15): New function.

gdb/testsuite/gdb.cp/oranking.cc

@@ -1,3 +1,18 @@
+#include <cstdlib>
+
+/* Make sure `malloc' is linked into the program.  If we don't, tests
+   in the accompanying expect file may fail:
+
+   evaluation of this expression requires the program to have a function
+   "malloc".  */
+
+void
+dummy ()
+{
+  void *p = malloc (16);
+
+  free (p);
+}
 
 /* 1. A standard covnersion sequence is better than a user-defined sequence
       which is better than an elipses conversion sequence.  */
@@ -165,6 +180,8 @@ test15 ()
 }
 
 int main() {
+  dummy ();
+
   B b;
   foo0(b);
   foo1(b);