core: ltc: add missing string_ext.h include

Add a missing include <string_ext.h> needed to fix:
core/lib/libtomcrypt/gcm.c: In function ‘crypto_aes_gcm_dec_final’:
core/lib/libtomcrypt/gcm.c:198:13: error: implicit declaration of function ‘consttime_memcmp’ [-Werror=implicit-function-declaration]
  198 |         if (consttime_memcmp(dst_tag, tag, tag_len) != 0)

Signed-off-by: Jens Wiklander <[email protected]>
Acked-by: Jerome Forissier <[email protected]>
Acked-by: Etienne Carriere <[email protected]>

core/lib/libtomcrypt/gcm.c

@@ -7,6 +7,7 @@
 #include <crypto/crypto.h>
 #include <crypto/crypto_impl.h>
 #include <stdlib.h>
+#include <string_ext.h>
 #include <string.h>
 #include <tee_api_types.h>
 #include <tomcrypt_private.h>