[lldb] [cmake] Fix typo in unittest directory path

Fix a typo in a11cd0d that resulted
in additional "}" in unittest directory path, e.g.:

    CMake Error at cmake/modules/LLDBStandalone.cmake:104 (add_subdirectory):
      add_subdirectory given source
      "/var/tmp/portage/dev-util/lldb-16.0.0_pre20221111/work/lldb/../third-party}/utils/unittest"
      which is not an existing directory.
    Call Stack (most recent call first):
      CMakeLists.txt:29 (include)

lldb/cmake/modules/LLDBStandalone.cmake

@@ -101,7 +101,7 @@ if(LLDB_INCLUDE_TESTS)
   # Build the gtest library needed for unittests, if we have LLVM sources
   # handy.
   if (EXISTS ${LLVM_THIRD_PARTY_DIR}/unittest AND NOT TARGET llvm_gtest)
-    add_subdirectory(${LLVM_THIRD_PARTY_DIR}}/utils/unittest third-party/unittest)
+    add_subdirectory(${LLVM_THIRD_PARTY_DIR}/utils/unittest third-party/unittest)
   endif()
   # LLVMTestingSupport library is needed for Process/gdb-remote.
   if (EXISTS ${LLVM_MAIN_SRC_DIR}/lib/Testing/Support