Fix non-MSVC C89 build

I've been using rlutil under Haiku, which uses an ancient compiler (GCC 2.95, C89) and I've had to make a few changes to rlutil.h to get it to compile. Fortunately they are minor and shouldn't really affect the quality of the code. The changes are:

- Replace `#pragma once` with classic header guards
- Forward declare `locate()` using `RLUTIL_INLINE` as well, also moving the definition of `RLUTIL_INLINE` near the top of the file
- Declare `struct timeval tv` at top of block