Raise the deployment target of IndexStoreDB so it can link against our XCTest.

--- a/Package.swift
+++ b/Package.swift
@@ -4,6 +4,7 @@ import PackageDescription

 let package = Package(
   name: "IndexStoreDB",
+  platforms: [.macOS("10.12")],
   products: [
     .library(
       name: "IndexStoreDB",