cmake: update to C++20

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I35b572197d191cfd907cd4f9b6ce01e632bd490a
diff --git a/libpfr/CMakeLists.txt b/libpfr/CMakeLists.txt
index 8957c9f..dfa0ad2 100644
--- a/libpfr/CMakeLists.txt
+++ b/libpfr/CMakeLists.txt
@@ -1,6 +1,6 @@
 cmake_minimum_required(VERSION 2.8.10 FATAL_ERROR)
 project(pfr CXX)
-set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_CXX_STANDARD 20)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 
 include(ExternalProject)