On Unix-like operating systems stdio.h is usually line-buffered. As
putStr uses fputs(3) from stdio.h internally, output will be written to
standard out after a newline character is written to the buffer. Since
the prompt does not contain a newline, it will only be written to
standard output after the user presses return. I encountered this issue
on Alpine Linux which uses musl libc (instead of glibc). However, I
believe this issue is likely also reproducible with glibc. This commit
fixes this issue by flushing standard output after writing the prompt to
it. Surprisingly, `src/Idris/IDEMode/REPL.idr` already does this
correctly, `src/Idris/REPL.idr` does not though.
