libstdc++: testsuite: cast getpid result

On vxworks, in kernel mode, getpid's return type is a pointer type, so
std::to_string on it fails overload resolution.  Restore the type cast
from the original patch that suggested adding the pid.


for  libstdc++-v3/ChangeLog

	* testsuite/util/testsuite_fs.h (nonexistent_path): Convert
	the getpid result to an integral type.

libstdc++-v3/testsuite/util/testsuite_fs.h

@@ -163,7 +163,7 @@ namespace __gnu_test
       file.resize(64);
     // The combination of random counter and PID should be unique for a given
     // run of the testsuite.
-    file += std::to_string(::getpid());
+    file += std::to_string((unsigned long) ::getpid());
     p = std::move(file);
     if (test_fs::exists(p))
       throw test_fs::filesystem_error("Failed to generate unique pathname", p,