ARROW-5473: [C++] Fix googletest_ep build failure on windows+ninja

Fix proposed by @rip-nsk

Closes #6816 from bkietz/5473-Build-failure-on-googlete

Authored-by: Benjamin Kietzman <bengilgit@gmail.com>
Signed-off-by: Wes McKinney <wesm+git@apache.org>

cpp/cmake_modules/ThirdpartyToolchain.cmake

@@ -1399,7 +1399,7 @@ macro(build_gtest)
     set(GTEST_CMAKE_CXX_FLAGS "${GTEST_CMAKE_CXX_FLAGS} -DGTEST_CREATE_SHARED_LIBRARY=1")
   endif()
 
-  set(GTEST_PREFIX "${CMAKE_CURRENT_BINARY_DIR}/googletest_ep-prefix/src/googletest_ep")
+  set(GTEST_PREFIX "${CMAKE_CURRENT_BINARY_DIR}/googletest_ep-prefix")
   set(GTEST_INCLUDE_DIR "${GTEST_PREFIX}/include")
 
   set(_GTEST_RUNTIME_DIR ${BUILD_OUTPUT_ROOT_DIRECTORY})