[COMMITTED PATCH] Add missing #include in get-rounding-mode.h

[Roland McGrath]

> Once the -Wundef fallout is resolved, I hope we can move to building with 
> -Werror by default (with some suitable policy about using 
> -Wno-error=whatever or -Wno-whatever when warnings, possibly depending on 
> the compiler version, seem hard to fix - probably including comments about 
> what the warnings are and what platforms / GCC versions they are seen 
> with, to help guide future review of such -Wno- settings; existing -Wno- 
> uses could also do with such review).

Agreed (in principle, with picayunities to be hashed out).