RISC-V: include cpufeature.h in cpufeature.c

RISC-V: include cpufeature.h in cpufeature.c

Automation complains:
warning: symbol '__pcpu_scope_misaligned_access_speed' was not declared. Should it be static?

cpufeature.c doesn't actually include the header of the same name, as it
had not previously used anything from it.
The per-cpu variable is declared there, so include it to silence the
complaints.

Fixes: 62a31d6e38bd ("RISC-V: hwprobe: Support probing of misaligned access performance")
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Evan Green <evan@rivosinc.com>
Link: https://lore.kernel.org/r/20230420-wound-gizzard-2b2b589d9bea@spud
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>