Fix build

The build was broken:

error: weak declaration of 'tensorflow::npd::kTfNextPluggableDeviceUseCApi' must be public

This is because weak implies static. Fix it by explicitly declare as extern.

PiperOrigin-RevId: 531785697

tensorflow/core/common_runtime/next_pluggable_device/next_pluggable_device_c_api_flag.cc

@@ -18,7 +18,7 @@ namespace tensorflow {
 namespace npd {
 
 // Define as a weak symbol so that it can be overridden when necessary.
-const bool kTfNextPluggableDeviceUseCApi __attribute__((weak)) = false;
+extern const bool kTfNextPluggableDeviceUseCApi __attribute__((weak)) = false;
 
 }  // namespace npd
 }  // namespace tensorflow