common/Thread: include cleanup

This header uses `std::string`, but not `std::system_error`.

Signed-off-by: Max Kellermann <[email protected]>

Author: MaxKellermann

src/common/Thread.h

@@ -17,8 +17,8 @@
 #define CEPH_THREAD_H
 
 #include <functional>
+#include <string>
 #include <string_view>
-#include <system_error>
 #include <thread>
 #include <cstring>