Mark liburing dependency skip as fb-only

Summary:
`ovr_config//os:linux-sgx` is a special target that matches a specific internal
toolchain, and is not relevant to OSS folly. By marking this as fb-only, OSS
folly properly links in liburing and buils for uring based targets start
working.

Reviewed By: yfeldblum

Differential Revision: D62681697

fbshipit-source-id: b6ed3e31236a95c0c94929b4b4fd10ce886e6709

folly/io/async/BUCK

@@ -592,7 +592,7 @@ cpp_library(
         "linux",
         select({
             "DEFAULT": ["fbsource//third-party/liburing:uring"],
-            "ovr_config//os:linux-sgx": [],
+            # @fb-only: "ovr_config//os:linux-sgx": [], 
         }),
     )],
 )