pk: adapt to new ECP_LIGHT symbol

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
diff --git a/include/mbedtls/pk.h b/include/mbedtls/pk.h
index 5d2cf34..c579661 100644
--- a/include/mbedtls/pk.h
+++ b/include/mbedtls/pk.h
@@ -771,7 +771,7 @@
 }
 #endif /* MBEDTLS_RSA_C */
 
-#if defined(MBEDTLS_ECP_C)
+#if defined(MBEDTLS_ECP_LIGHT)
 /**
  * Quick access to an EC context inside a PK context.
  *
@@ -794,7 +794,7 @@
             return NULL;
     }
 }
-#endif /* MBEDTLS_ECP_C */
+#endif /* MBEDTLS_ECP_LIGHT */
 
 #if defined(MBEDTLS_PK_PARSE_C)
 /** \ingroup pk_module */