sgx-tools/README: add the libsgx-aesm-quote-ex-plugin dependency.
Fixes:
- golang 1.14 or above.
- libsgx-dcap-quote-verify-devel (CentOS) / libsgx-dcap-quote-verify-dev (Ubuntu) from [official Intel SGX DCAP release page](https://01.org/intel-software-guard-extensions/downloads).
+- libsgx-aesm-quote-ex-plugin from [official Intel SGX Linux release page](https://01.org/intel-software-guard-extensions/downloads).
## Build
