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/buffer.h

@@ -1,5 +1,4 @@
-#ifndef NVIM_BUFFER_H
-#define NVIM_BUFFER_H
+#pragma once
 
 #include <assert.h>
 #include <stdbool.h>
@@ -140,5 +139,3 @@ static inline bool buf_is_empty(buf_T *buf)
   return buf->b_ml.ml_line_count == 1
          && *ml_get_buf(buf, 1) == '\0';
 }
-
-#endif  // NVIM_BUFFER_H