Improve the include situation

Slightly clean up header inclusion with the help of clang's module system.
It's better for files to include the things you need directly.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/backend/backend_common.c

@@ -2,6 +2,7 @@
 
 #include "backend.h"
 #include "backend_common.h"
+#include "kernel.h"
 #include "common.h"
 #include "x.h"