Fix build error when CROSS_COMPILE is not set

If the CROSS_COMPILE environment variable is not set, incorrect
quotation in cmake script causes build error when building
mbedtls.
This change adds the missing quotes and fixes the issue.

Change-Id: Ic7817c318c7ce9ac76d8c58f86070063e28ef42c
Signed-off-by: Gyorgy Szing <Gyorgy.Szing@arm.com>
1 file changed