libtomcrypt: define LTC_MPI at the same time as LTC_DER

LTC_DER is always defined when LTC_NO_PKCS is NOT defined (which is our
case). Since LTC_DER depends on LTC_MPI let's set that one at the same
time. Fixes the following build error:

 $ make CFG_CRYPTO=n
 [...]
 core/lib/libtomcrypt/src/headers/tomcrypt_custom.h:629:5: error: #error ASN.1 DER requires MPI functionality
   629 |    #error ASN.1 DER requires MPI functionality
       |     ^~~~~

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>

core/lib/libtomcrypt/src/headers/tomcrypt_custom.h

@@ -485,6 +485,8 @@
 /* Include ASN.1 DER (required by DSA/RSA) */
 #define LTC_DER
 
+#define LTC_MPI
+
 #endif /* LTC_NO_PKCS */
 
 /* misc stuff */
@@ -574,9 +576,6 @@
 #endif
 
 #if defined(LTC_MECC) || defined(LTC_MRSA) || defined(LTC_MDSA) || defined(LTC_SSH)
-   /* Include the MPI functionality?  (required by the PK algorithms) */
-   #define LTC_MPI
-
    #ifndef LTC_PK_MAX_RETRIES
       /* iterations limit for retry-loops */
       #define LTC_PK_MAX_RETRIES  20