illumos: sendfile(3EXT) is in libsendfile, not libc

As per https://illumos.org/man/3EXT/sendfile a separate header and
library are required to access sendfile() and sendfilev() on illumos
systems.

libc-test/build.rs

@@ -781,6 +781,7 @@ fn test_solarish(target: &str) {
         "sys/priv.h",
         "sys/pset.h",
         "sys/resource.h",
+        "sys/sendfile.h",
         "sys/socket.h",
         "sys/stat.h",
         "sys/statvfs.h",

src/unix/solarish/mod.rs

@@ -2817,7 +2817,10 @@ extern "C" {
 
     pub fn getpflags(flags: ::c_uint) -> ::c_uint;
     pub fn setpflags(flags: ::c_uint, value: ::c_uint) -> ::c_int;
+}
 
+#[link(name = "sendfile")]
+extern "C" {
     pub fn sendfile(out_fd: ::c_int, in_fd: ::c_int, off: *mut ::off_t, len: ::size_t)
         -> ::ssize_t;
     pub fn sendfilev(