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

Similar to 79f24568, 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: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20180628153535.1411-5-f4bug@amsat.org>
Signed-off-by: NFam Zheng <famz@redhat.com>

tests/vm/ubuntu.i386

@@ -77,7 +77,7 @@ class UbuntuX86VM(basevm.BaseVM):
         # The previous update sometimes doesn't survive a reboot, so do it again
         self.ssh_root_check("apt-get update")
         self.ssh_root_check("apt-get build-dep -y qemu")
-        self.ssh_root_check("apt-get install -y libfdt-dev")
+        self.ssh_root_check("apt-get install -y libfdt-dev flex bison")
         self.ssh_root("poweroff")
         self.wait()
         if os.path.exists(img):