refactor: replace manual header guards with #pragma once

It is less error-prone than manually defining header guards. Pretty much
all compilers support it even if it's not part of the C standard.

src/nvim/channel.h

@@ -1,5 +1,4 @@
-#ifndef NVIM_CHANNEL_H
-#define NVIM_CHANNEL_H
+#pragma once
 
 #include <stdbool.h>
 #include <stdint.h>
@@ -161,5 +160,3 @@ static inline Stream *channel_outstream(Channel *chan)
   }
   abort();
 }
-
-#endif  // NVIM_CHANNEL_H