fix incorrect signature for ofGetTimestampString

This looks like the last regression from #2914 . I flipped on "Missing Function Prototypes" in Xcode, which found this one and no others in `ofUtils.h`, so that should be it.

Aside: there's a few internal OF functions which throw warnings for this (`ofGLReadyCallback()`, `ofURLFileLoaderShutdown()`, `ofInitFreeImage()`) but nothing in the OF headers.

ping @kylemcdonald , @bakercp