add unistd_h (#2959)

* add unistd_h

* Update xmake.lua

packages/u/unistd_h/xmake.lua

@@ -0,0 +1,16 @@
+package("unistd_h")
+    set_kind("library", {headeronly = true})
+    set_homepage("https://github.com/win32ports/unistd_h")
+    set_description("header-only Windows implementation of the <unistd.h> header")
+    set_license("MIT")
+
+    add_urls("https://github.com/win32ports/unistd_h.git")
+    add_versions("2019.07.30", "0dfc48c1bc67fa27b02478eefe0443b8d2750cc2")
+
+    on_install("windows", function (package)
+        os.cp("unistd.h", package:installdir("include"))
+    end)
+
+    on_test(function (package)
+        assert(package:has_cfuncs("access", {includes = "unistd.h"}))
+    end)