Include stddef.h for size_t

 Headers should compile as if they were source code too. Without including stddef.h, linenoise.h cannot be included without first including something that exposes size_t. stddef.h is the least invasive way of doing this, other headers like stdio.h and string.h; while they may expose size_t are more invasive. This patch also fixes trailing whitespace in the license header as well.