Fix macOS Firestore linker error by adding Network framework dependency

Author: AustinBenoit

firestore/integration_test/CMakeLists.txt

@@ -207,6 +207,7 @@ else()
       "-framework CoreFoundation"
       "-framework Foundation"
       "-framework GSS"
+      "-framework Network"
       "-framework Security"
       "-framework SystemConfiguration"
     )

firestore/integration_test_internal/CMakeLists.txt

@@ -371,6 +371,7 @@ else()
       "-framework CoreFoundation"
       "-framework Foundation"
       "-framework GSS"
+      "-framework Network"
       "-framework Security"
       "-framework SystemConfiguration"
     )