channel: Fix redundant redeclaration warning

channel: Fix redundant redeclaration warning

g++ reports:
```
In file included from channel.cc:23:
channel.hh:38:17: warning: redundant redeclaration of ‘void __tsan_acquire(void*)’ in same scope [-Wredundant-decls]
   38 | extern "C" void __tsan_acquire(void* addr);
      |                 ^~~~~~~~~~~~~~
In file included from /usr/include/c++/13.2.1/bits/shared_ptr_atomic.h:37,
                 from /usr/include/c++/13.2.1/memory:81,
                 from channel.hh:23:
/usr/lib/gcc/x86_64-pc-linux-gnu/13.2.1/include/sanitizer/tsan_interface.h:24:6: note: previous declaration of ‘void __tsan_acquire(void*)’
   24 | void __tsan_acquire(void *addr);
      |      ^~~~~~~~~~~~~~
channel.hh:39:17: warning: redundant redeclaration of ‘void __tsan_release(void*)’ in same scope [-Wredundant-decls]
   39 | extern "C" void __tsan_release(void* addr);
      |                 ^~~~~~~~~~~~~~
/usr/lib/gcc/x86_64-pc-linux-gnu/13.2.1/include/sanitizer/tsan_interface.h:25:6: note: previous declaration of ‘void __tsan_release(void*)’
   25 | void __tsan_release(void *addr);
```