Make the aes xts methods independent from MBEDTLS_AES_SETKEY_DEC_ALT

This is fix for the MBEDTLS_AES_SETKEY_DEC_ALT macro switch is including the aes xts methods
and building with a custom mbedtls_aes_setkey_dec function will disable the aes xts methods.
The fix is separating the aes xts methods and the MBEDTLS_AES_SETKEY_DEC_ALT can only
switch the presence of the mbedtls_aes_setkey_dec function.

Signed-off-by: gabor-mezei-arm <gabor.mezei@arm.com>
diff --git a/library/aes.c b/library/aes.c
index ed48b24..b538664 100644
--- a/library/aes.c
+++ b/library/aes.c
@@ -730,6 +730,7 @@
 
     return( ret );
 }
+#endif /* !MBEDTLS_AES_SETKEY_DEC_ALT */
 
 #if defined(MBEDTLS_CIPHER_MODE_XTS)
 static int mbedtls_aes_xts_decode_keys( const unsigned char *key,
@@ -808,8 +809,6 @@
 }
 #endif /* MBEDTLS_CIPHER_MODE_XTS */
 
-#endif /* !MBEDTLS_AES_SETKEY_DEC_ALT */
-
 #define AES_FROUND(X0,X1,X2,X3,Y0,Y1,Y2,Y3)                     \
     do                                                          \
     {                                                           \