Include <sys/uio.h> unconditionally

Include <sys/uio.h> unconditionally

Since <sys/uio.h> is standardized by POSIX and is present in all
available versions of glibc, it's safe to assume that any usable
libc implementation provides this header file.

* configure.ac (AC_CHECK_HEADERS): Remove sys/uio.h.
* io.c: Include <sys/uio.h> unconditionally.
(tprint_iov_upto, tprint_iov, sys_readv, sys_writev,
print_llu_from_low_high_val, sys_preadv, sys_pwritev): Define
unconditionally.
* net.c: Include <sys/uio.h> unconditionally.
* util.c: Include <sys/uio.h> unconditionally.
(dumpiov): Define unconditionally.