BUILD: buffers: buf.h requires unistd to get ssize_t on libmusl

BUILD: buffers: buf.h requires unistd to get ssize_t on libmusl

Building with musl and gcc-5.3 for MIPS returns this :

include/common/buf.h: In function 'b_dist':
include/common/buf.h:252:2: error: unknown type name 'ssize_t'
  ssize_t dist = to - from;
  ^
Including stdint or stddef is not sufficient there to get ssize_t,
unistd is needed as well. It's likely that other platforms will have
the same issue. This patch also addresses it in ist.h and memory.h.