include: zephyr: linker: linker-devnull: Fix include-guard

The implementation of include-guard is incomplete, so I fixed it.

Signed-off-by: TOKITA Hiroshi <[email protected]>

Author: soburi

include/zephyr/linker/linker-devnull.h

@@ -11,6 +11,7 @@
  */
 
 #ifndef ZEPHYR_INCLUDE_LINKER_LINKER_DEVNULL_H_
+#define ZEPHYR_INCLUDE_LINKER_LINKER_DEVNULL_H_
 
 #if defined(CONFIG_LINKER_DEVNULL_MEMORY)