Fix ifdef conditions for EC-related extensions.

Was alternatively ECP_C and ECDH_C.
diff --git a/include/polarssl/ssl.h b/include/polarssl/ssl.h
index 0e3047f..7a468c4 100644
--- a/include/polarssl/ssl.h
+++ b/include/polarssl/ssl.h
@@ -428,7 +428,9 @@
 #if defined(POLARSSL_ECDH_C)
     ecdh_context ecdh_ctx;              /*!<  ECDH key exchange       */
 #endif
+#if defined(POLARSSL_ECDH_C) || defined(POLARSSL_ECDSA_C)
     int ec_curve;                       /*!<  Selected elliptic curve */
+#endif
 
     /*
      * Checksum contexts