selftests/rseq: riscv: fix 'literal-suffix' warning

This header is also used in librseq where it can be included in C++
code, add a space between literals and string macros.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://lore.kernel.org/r/20220614154830.1367382-3-mjeanson@efficios.com