update googletest version to 1.8.1

cmake/external_libs/gtest.cmake

@@ -11,13 +11,13 @@ if ((${CMAKE_INSTALL_PREFIX} STREQUAL /usr/local) OR
 endif()
 
 if (GE_PB_PKG)
-    set(REQ_URL "${GE_PB_PKG}/libs/ge_gtest/release-1.8.0.tar.gz")
+    set(REQ_URL "${GE_PB_PKG}/libs/ge_gtest/release-1.8.1.tar.gz")
     set(MD5 "")
 elseif (ENABLE_GITEE)
-    set(REQ_URL "https://gitee.com/mirrors/googletest/repository/archive/release-1.8.0.tar.gz")
+    set(REQ_URL "https://gitee.com/mirrors/googletest/repository/archive/release-1.8.1.tar.gz")
     set(MD5 "")
 else()
-    set(REQ_URL "https://github.com/google/googletest/archive/release-1.8.0.tar.gz")
+    set(REQ_URL "https://github.com/google/googletest/archive/release-1.8.1.tar.gz")
     set(MD5 "")
 endif ()