build: ensure C++ std is set

The unit tests files are C++ and this change
sets up the C++ std for the compiler.

Change-Id: Id94009b7a2041893bbecf31f94d9ddd67b88511f
Signed-off-by: Soby Mathew <soby.mathew@arm.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 2b01830..0d94f41 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -60,6 +60,8 @@
 
 set(CMAKE_C_STANDARD 11)
 set(CMAKE_C_STANDARD_REQUIRED TRUE)
+set(CMAKE_CXX_STANDARD 11)
+set(CMAKE_CXX_STANDARD_REQUIRED TRUE)
 set(CMAKE_C_EXTENSIONS TRUE)
 
 if(RMM_STATIC_ANALYSIS_CPPCHECK)