package/mtd: crypto support needs headers >= 4.12

package/mtd: crypto support needs headers >= 4.12

crypto support includes <linux/hash_info.h> since version 2.1.2 and
https://github.com/sigma-star/mtd-utils/commit/a739b59efe7996e3bdcbe8b17743dc05ac7c110a

However, linux/hash_info.h is only exported since kernel 4.12:
https://github.com/torvalds/linux/commit/fcc8487d477a3452a1d0ccbdd4c5e0e1e3cb8bed

Fixes:
 - http://autobuild.buildroot.org/results/dbaeed9e9e2af191166808d2d6d6d3c2e5051c2c

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>