perf riscv: Fix the warning due to the incompatible type

In the 32-bit platform, the second argument of getline is expectd to be
'size_t *'(aka 'unsigned int *'), but line_sz is of type
'unsigned long *'. Therefore, declare line_sz as size_t.

Signed-off-by: Ben Zong-You Xie <ben717@andestech.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20240305120501.1785084-3-ben717@andestech.com


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