pk: adapt to new ECP_LIGHT symbol

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
diff --git a/library/pk_wrap.h b/library/pk_wrap.h
index c5cd4df..89bce40 100644
--- a/library/pk_wrap.h
+++ b/library/pk_wrap.h
@@ -120,7 +120,7 @@
 extern const mbedtls_pk_info_t mbedtls_rsa_info;
 #endif
 
-#if defined(MBEDTLS_ECP_C)
+#if defined(MBEDTLS_ECP_LIGHT)
 extern const mbedtls_pk_info_t mbedtls_eckey_info;
 extern const mbedtls_pk_info_t mbedtls_eckeydh_info;
 #endif