tracing: actually run global_subscriber benchmark

Seems like this was never actually being run due me being dumb and
forgetting to add it to the Cargo.toml. Whoops!

Author: hawkw

tracing/Cargo.toml

@@ -75,6 +75,10 @@ harness = false
 name = "no_subscriber"
 harness = false
 
+[[bench]]
+name = "global_subscriber"
+harness = false
+
 [badges]
 maintenance = { status = "actively-developed" }