target/riscv: Add missing include guard in pmu.h

target/riscv: Add missing include guard in pmu.h

Add missing include guard in pmu.h to avoid the problem of double
inclusion.

Signed-off-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20240220110907.10479-1-frank.chang@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>