[stdlib] Add missing import to path.vale

[zupat]

Used here:

Vale/stdlib/src/path/path.vale

Line 137 in 02f39df

new_segments = segment.split(GetPathSeparator());

Error without it:

Error: stdlib.path:/home/dev/programs/vale_0.2.0.28/stdlib/src/path/path.vale:134:31 error T
  new_segments = segment.split(GetPathSeparator());
Couldn't find a suitable function split(str, str). No function with that name exists.