syscalls: add 'pragma export' to script that generates header

Otherwise it gets lost every time the header is regenerated

Follow-up for 3111327ca4

src/include/override/sys/generate-syscall.py

@@ -45,7 +45,7 @@ HEADER = '''\
  */
 #pragma once
 
-#include_next <sys/syscall.h>
+#include_next <sys/syscall.h>   /* IWYU pragma: export */
 
 #ifdef ARCH_MIPS
 #include <asm/sgidefs.h>