Increase timeout for performance benchmarks

Fix a CI failure that is also observed in rust-vmm/event-manager
See also rust-vmm/event-manager#101

Signed-off-by: Patrick Roy <[email protected]>

Author: roypat

.buildkite/bench-tests.json

@@ -6,7 +6,8 @@
             "platform": [
                 "x86_64",
                 "aarch64"
-            ]
+            ],
+            "timeout_in_minutes": 15
         }
     ]
 }