sim: added missing include to mathexpr.hh

sim: added missing include to mathexpr.hh

mathexpr.hh uses std::function<> but was not including the appropriate
header, which resulted in an error

build/ARM/sim/mathexpr.hh:51:18: error: 'function' in namespace 'std'
does not name a template type
     typedef std::function<double(std::string)> EvalCallback;

This commit adds the missing include.

Change-Id: I6c01d77d4354c6de838538f137a38f75f9866166
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>