bootutil/crypto: Fix the common.h header

The crypto/common.h header checks for MBEDTLS_VERSION_NUMBER
value but it needs to include mbedtls/version.h first
otherwise it won't return a reliable check.

Signed-off-by: Antonio de Angelis <Antonio.deAngelis@arm.com>
Change-Id: Ice12fe26bb24fd98c09c4adfe001b5274cee555c