build: avoid a signed overflow warning in ptx

build: avoid a signed overflow warning in ptx

* src/ptx.c (fix_output_parameters): GCC 6.3.1 with
./configure --enable-single-binary would give:
  error: assuming signed overflow does not occur
  when simplifying conditional to constant [-Werror=strict-overflow]
    if (file_index > 0)
So change the type of file_index to signed (size_t).