Add StoreKit framework to Firebase build file.

Firebase now depends on StoreKit, so it's necessary to link to it
to be able to compile with Firebase. This should fix the compilation
downstream on iOS.

Bug: none
Test: none
Change-Id: I988a078e38996ed5895739e3b20061a760333cfc
Reviewed-on: https://chromium-review.googlesource.com/c/1470196
Reviewed-by: Olivier Robin <[email protected]>
Reviewed-by: Mark Cogan <[email protected]>
Commit-Queue: Stepan Khapugin <[email protected]>
Cr-Commit-Position: refs/heads/master@{#631713}

Author: Stepan Khapugin

ios/third_party/firebase/BUILD.gn

@@ -29,5 +29,6 @@ config("firebase_config") {
     "GoogleAppMeasurement.framework",
     "GoogleUtilities.framework",
     "nanopb.framework",
+    "StoreKit.framework",
   ]
 }