mmc: core: Add a missing SPDX license header

Add the missing license header to drivers/mmc/core/block.c
Signed-off-by: Aviral Gupta <shiv14112001@gmail.com>
Link: https://lore.kernel.org/r/20210523155914.5200-1-shiv14112001@gmail.comSigned-off-by: Ulf Hansson <ulf.hansson@linaro.org>

drivers/mmc/core/block.c

+// SPDX-License-Identifier: GPL-2.0
 /*
  * Block driver for media (i.e., flash cards)
  *