riscv: remove unused header

arch/riscv/kernel/sys_riscv.c builds without using anything
from asm-generic/mman-common.h. There seems to be no reason
to include this file.

Signed-off-by: Tanzir Hasan <tanzirh@google.com>
Fixes: 9e2e6042 ("riscv: Allow PROT_WRITE-only mmap()")
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Link: https://lore.kernel.org/r/20231212-removeasmgeneric-v2-1-a0e6d7df34a7@google.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>