build: fix --enable-shared vout linking error on iOS

This is a follow up MR on !4271 (merged)

It fixes the following error:

Undefined symbols for architecture arm64:
  "_OBJC_CLASS_$_NSNumber", referenced from:
      objc-class-ref in libsamplebufferdisplay_plugin_la-VLCSampleBufferDisplay.o

when building iOS with --enable-shared