lkl: hijack: explicitly build with -std=gnu11

HOST_CALL() uses a (*host_##name)() function pointer while callers
provide regular syscall parameters. With gcc -std=gnu23 this results in:
lib/hijack/hijack.c: In function ‘hijack_setsockopt’:
lib/hijack/hijack.c:176:24: error: too many arguments to function ‘host_setsockopt’; expected 0, have 5
  176 |                 return host_setsockopt(fd, level, optname, optval, optlen);
      |                        ^~~~~~~~~~~~~~~ ~~

Signed-off-by: David Disseldorp <ddiss@suse.de>

tools/lkl/lib/hijack/Build

@@ -9,3 +9,6 @@ liblkl-zpoline-y += hijack.o
 liblkl-zpoline-y += init.o
 liblkl-zpoline-y += xlate.o
 liblkl-zpoline-y += dbg_handler.o
+
+# -std=gnu23/c23 fails due to HOST_CALL (*host_##name)()
+CFLAGS_hijack.o += -std=gnu11