xtensa_lx60: add missing #include "blockdev.h"

xtensa_lx60: add missing #include "blockdev.h"

This should fix the following build failure:

/home/buildbot/slave-public/block_mingw32/build/hw/xtensa_lx60.c: In function 'lx_init':
/home/buildbot/slave-public/block_mingw32/build/hw/xtensa_lx60.c:212: warning: implicit declaration of function 'drive_get'
/home/buildbot/slave-public/block_mingw32/build/hw/xtensa_lx60.c:212: warning: nested extern declaration of 'drive_get'
/home/buildbot/slave-public/block_mingw32/build/hw/xtensa_lx60.c:212: error: 'IF_PFLASH' undeclared (first use in this function)
/home/buildbot/slave-public/block_mingw32/build/hw/xtensa_lx60.c:212: error: (Each undeclared identifier is reported only once
/home/buildbot/slave-public/block_mingw32/build/hw/xtensa_lx60.c:212: error: for each function it appears in.)
/home/buildbot/slave-public/block_mingw32/build/hw/xtensa_lx60.c:216: error: dereferencing pointer to incomplete type

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>