Fix build of cpptest by correcting relative include paths

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 757b0d1a8..546ecd516 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -778,6 +778,11 @@ endif()
 if(GTEST_FOUND)
   tvm_file_glob(GLOB_RECURSE TEST_SRCS tests/cpp/*.cc)
   add_executable(cpptest ${TEST_SRCS})
+
+  # fix building cpptest if we don't use 3rdParty subtree as when building with GNU Guix
+  # allowng relative paths starting with ../../.. to point to the top of the source dir
+  target_include_directories(cpptest PRIVATE tests/cpp/runtime)
+
   # include runtime files for unit testing
   target_link_libraries(cpptest PRIVATE ${TVM_TEST_LIBRARY_NAME} GTest::GTest GTest::Main GTest::gmock pthread dl)
   if(DEFINED LLVM_LIBS)