`SSLUtil` does not check whether Folly has uring support

[sin-ack]

In this bit of code, the check is done by directly checking whether a header is present:

fbthrift/thrift/lib/cpp2/security/SSLUtil.cpp

Line 136 in 7560ce1

#if defined(__linux__) && __has_include(<liburing.h>)

However, this doesn't check whether Folly was compiled with uring support. Folly already has a feature test header for this purpose, please use it instead of this ad-hoc check.