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