Update manifest for s2n feature probe (#718)

Author: xiazhvera

MANIFEST.in

@@ -32,6 +32,8 @@ prune crt/aws-c-sdkutils/tests
 prune crt/aws-c-cal/ecdsa-fuzz-corpus
 prune crt/aws-c-s3/benchmarks
 prune crt/s2n/tests
+# DO NOT REMOVE!! s2n's cmake relies on files under tests/ for compile time feature probing
+graft crt/s2n/tests/features
 prune crt/s2n/compliance/specs
 exclude crt/aws-lc/**/*test*.go
 exclude crt/aws-lc/**/*test*.json