Linux: Forward declaration of struct iovec for process_madvise

This maintains compatibility between <sys/mman.h> and <linux/uio.h>.
Before that, the addition of process_madvise made those two header
files incompatible.  This has been observed resulting in a build
failure in LLDB's Process/Linux/NativeRegisterContextLinux_s390x.cpp
source file.

Fixes commit d19ee347
("linux: Add process_madvise").

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>