coin: Enable target-protoc for vcpkg-provided protobuf

This feature enables us to build protobuf with different host/target
triplets to lipo the result.

Fixes: QTBUG-145933
Change-Id: I1456c148686c27cecc6f1f2f5decdeb6f0a2304d
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

coin/provisioning/common/shared/vcpkg/vcpkg.json

@@ -8,7 +8,15 @@
   ],
   "dependencies": [
     "openssl",
-    "protobuf",
+    {
+      "name": "protobuf",
+      "platform": "!osx"
+    },
+    {
+      "name": "protobuf",
+      "platform": "osx",
+      "features": [ "target-protoc" ]
+    },
     "grpc"
   ]
 }