[libc] Fix build failure with ASSUME_ROUND_NEAREST_ONLY (#207991)
Commit 03c62ca40d19ba0 moved some local variables of
`convert_float_hex_exp` into a struct called `properties`, but didn't
edit the #ifdef branch for `LIBC_MATH_HAS_ASSUME_ROUND_NEAREST_ONLY`, so
builds with that definition failed.