riscv: tlb: Provide definition of tlb_flush() before including tlb.h

As of commit fd1102f0 ("mm: mmu_notifier fix for tlb_end_vma"),
asm-generic/tlb.h now calls tlb_flush() from a static inline function,
so we need to make sure that it's declared before #including the
asm-generic header in the arch header.

Reported-by: Guenter Roeck <linux@roeck-us.net>
Fixes: fd1102f0 ("mm: mmu_notifier fix for tlb_end_vma")
Signed-off-by: Will Deacon <will.deacon@arm.com>
[groeck: Use forward declaration instead of moving inline function]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>