tests/vm: Add flex and bison to the vm image

tests/vm: Add flex and bison to the vm image

Similar to 79f24568e5e70, this fixes the following warnings:

           CHK version_gen.h
           LEX convert-dtsv0-lexer.lex.c
  make[1]: flex: Command not found
           BISON dtc-parser.tab.c
  make[1]: bison: Command not found
           LEX dtc-lexer.lex.c
  make[1]: flex: Command not found

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20180628153535.1411-5-f4bug@amsat.org>
Signed-off-by: Fam Zheng <famz@redhat.com>