include: riscv: Fix wrong include guard in riscv_pmu.h

The current include guard prevents the inclusion of asm/perf_event.h
which uses the same include guard: fix the one in riscv_pmu.h so that it
matches the file name.

Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>