Fix cosmopolitan.h for C++

We now have an integration test that runs the amalgamated sources
through a C++ compiler, to prevent the recurrence of such issues.

Fixes #38
Thanks @gbbnfhb for the report!

test/libc/release/smokecxx.cc

@@ -0,0 +1,11 @@
+int main() {
+  int rc;
+  FILE *f;
+  f = fopen("/dev/null", "w");
+  fprintf(f, "hello world\n");
+  fclose(f);
+  rc = system("exit 42");
+  CHECK_NE(-1, rc);
+  CHECK_EQ(42, WEXITSTATUS(rc));
+  return 0;
+}