Fix external_path_test with newer Xcode versions.

I found this when running Bazel's test suite with Xcode 12.2:

```
external/remote/lib/lib.c:2:3: error: implicitly declaring library function 'printf' with type 'int (const char *, ...)' [-Werror,-Wimplicit-function-declaration]
  printf("Hello %s\n", s);
  ^
external/remote/lib/lib.c:2:3: note: include the header <stdio.h> or explicitly provide a declaration for 'printf'
```

PiperOrigin-RevId: 344247362

src/test/shell/bazel/external_path_test.sh

@@ -69,6 +69,8 @@ EOF
 #define TARGET "World"
 EOF
   cat > lib/lib.c <<'EOF'
+#include <stdio.h>
+
 int greet(char *s) {
   printf("Hello %s\n", s);
   return 0;