Add limits header to ensure gcc11 compatibility

Since gcc11 some headers (including limts) are used much less in the standard library headers. See https://stackoverflow.com/questions/4798936/numeric-limits-was-not-declared-in-this-scope-no-matching-function-for-call-t

Author: TobiKattmann

SU2_CFD/include/variables/CEulerVariable.hpp

@@ -27,6 +27,7 @@
 
 #pragma once
 
+#include <limits>
 #include "CFlowVariable.hpp"
 
 /*!

SU2_CFD/include/variables/CIncEulerVariable.hpp

@@ -27,6 +27,7 @@
 
 #pragma once
 
+#include <limits>
 #include "CFlowVariable.hpp"
 
 /*!