Two minor fixes to enable timing and allow compilation in IAR Embedded Workbench

As discussed on the forum at:

https://groups.google.com/d/topic/cpputest/WxCfnVZYGHw/discussion

I believe these are necessary for IAR support, and are low risk impact in general. Caveat: this is my first pull request.

Suggested commit message:

Bug fix to Iar/UtestPlatform to enable timing.
Explicit casts added to tests/AllocLetTestFree.c to allow compilation in IAR.