Define UINT64_C when needed.
Define UINT64_C when needed.

rate.c now uses this, but it is normally defined by stdint.h, which
 is not always available.
Fixes a compilation problem on MSVC 2008 or earlier.

luctrudeau/daala,luctrudeau/daala,luctrudeau/daala,ycho/daala,luctrudeau/daala,xiph/daala,ycho/daala,xiph/daala,luctrudeau/daala,xiph/daala,ycho/daala,ycho/daala,xiph/daala,ycho/daala,xiph/daala