[flang][cuda] Fix char argument

This would fail with `error: argument of type "char" is incompatible with parameter of type "const char *"`
diff --git a/flang-rt/lib/runtime/stop.cpp b/flang-rt/lib/runtime/stop.cpp
index a4ef710..69a99e6 100644
--- a/flang-rt/lib/runtime/stop.cpp
+++ b/flang-rt/lib/runtime/stop.cpp
@@ -80,7 +80,7 @@
     if (code != EXIT_SUCCESS) {
       std::printf(": code %d\n", code);
     }
-    std::printf('\n');
+    std::printf("\n");
   }
 #if defined(__CUDACC__)
   // NVCC supports __trap().