fbset: add dependency flex-native

Build fbset failed with:
| flex modes.l
| make: flex: Command not found
| make: *** [lex.yy.c] Error 127

Add dependency flex-native to fix it.

Signed-off-by: Kai Kang <kai.kang@windriver.com>
Signed-off-by: Jackie Huang <jackie.huang@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>