leveldb.cmake: add URL_HASH to improve security (#976)

Author: dconeybe

cmake/external/leveldb.cmake

@@ -26,6 +26,7 @@ ExternalProject_Add(
   DOWNLOAD_DIR ${FIREBASE_DOWNLOAD_DIR}
   DOWNLOAD_NAME leveldb-${version}.tar.gz
   URL https://github.com/google/leveldb/archive/${version}.tar.gz
+  URL_HASH SHA256=9a37f8a6174f09bd622bc723b55881dc541cd50747cbd08831c2a82d620f6d76
 
   PREFIX ${PROJECT_BINARY_DIR}