Change c++ std to 20

Building things that use sdbusplus with gcc-12 requires
-std=c++-20 to successfully compile.

Change-Id: Ie8dec803e4cbaa18c2c5b51672c01680363b68f8
Signed-off-by: Vernon Mauery <vernon.mauery@linux.intel.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 63241c5..97282be 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -3,6 +3,7 @@
 
 add_library(peci SHARED peci.c)
 
+set(CMAKE_CXX_STANDARD 20)
 set_property(TARGET peci PROPERTY C_STANDARD 99)
 target_include_directories(peci PUBLIC ${CMAKE_CURRENT_SOURCE_DIR})
 set_target_properties(peci PROPERTIES VERSION "1.0" SOVERSION "1")