[libc] Fix undeclared 'free' function in stream test

Summary:
We need this function from the test.cpp but need to declare it manually.

Author: jhuber6

libc/test/integration/startup/gpu/rpc_stream_test.cpp

@@ -15,6 +15,7 @@
 #include "test/IntegrationTest/test.h"
 
 extern "C" void *malloc(uint64_t);
+extern "C" void free(void *);
 
 using namespace __llvm_libc;