mmc: fix compilation of user API

mmc: fix compilation of user API

The definitions of MMC_IOC_CMD  and of MMC_IOC_MULTI_CMD rely on
MMC_BLOCK_MAJOR:

    #define MMC_IOC_CMD       _IOWR(MMC_BLOCK_MAJOR, 0, struct mmc_ioc_cmd)
    #define MMC_IOC_MULTI_CMD _IOWR(MMC_BLOCK_MAJOR, 1, struct mmc_ioc_multi_cmd)

However, MMC_BLOCK_MAJOR is defined in linux/major.h and
linux/mmc/ioctl.h did not include it.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20200511161902.191405-1-Jerome.Pouiller@silabs.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>