riscv: Fix compilation error with FAST_GUP and rv32

riscv: Fix compilation error with FAST_GUP and rv32

By surrounding the definition of pte_leaf_size() with a ifdef napot as
it should have been.

Fixes: e0fe5ab4192c ("riscv: Fix pte_leaf_size() for NAPOT")
Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Link: https://lore.kernel.org/r/20240304080247.387710-1-alexghiti@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>