Add seapp_contexts to allowlist of makefile goal

This is an interim solution before seapp_contexts and seapp_neverallows
are migrated into Soong. CtsSecurityHostTests needs these two modules.

Bug: 194096505
Test: build
Change-Id: I5596d6f00b957afd86c752501c1e83b3d4089d9e

android/neverallow_test.go

@@ -293,7 +293,7 @@ var neverallowTests = []struct {
 			`),
 		},
 		expectedErrors: []string{
-			"Only boot images may be imported as a makefile goal.",
+			"Only boot images and seapp contexts may be imported as a makefile goal.",
 		},
 	},
 }