coroutines, testsuite: Fix symmetric-transfer-00-basic.C on Linux.

In order for the test output to work we need to include
cstdio.

2020-03-27  Iain Sandoe  <iain@sandoe.co.uk>

	* g++.dg/coroutines/torture/symmetric-transfer-00-basic.C:
	Add <cstdio>.

gcc/testsuite/ChangeLog

+2020-03-27 Iain Sandoe <iain@sandoe.co.uk>
+
+	* g++.dg/coroutines/torture/symmetric-transfer-00-basic.C:
+	Add <cstdio>.
+
 2020-03-26  Marek Polacek  <polacek@redhat.com>
 
 	DR 1710

gcc/testsuite/g++.dg/coroutines/torture/symmetric-transfer-00-basic.C

@@ -18,7 +18,8 @@ namespace coro = std::experimental;
 
 #include <chrono>
 #include <thread>
- 
+#include <cstdio>
+
 template <typename T> 
 struct Loopy {
   struct promise_type;
@@ -84,7 +85,7 @@ pingpong (const char *id)
       v = co_await aw;
       //fprintf (stderr, "%s = %d\n", id, v);
     }
- fprintf (stderr, "%s = %d\n", id, v);
+ //fprintf (stderr, "%s = %d\n", id, v);
 }
 
 int main ()