patch 8.0.1568: can't build on older Mac, header file is missing

Problem:    Can't build on older Mac, header file is missing.
Solution:   Remove the header file. (Ozaki Kiichi, closes #2691)

src/os_unix.c

@@ -566,7 +566,6 @@ mch_check_messages(void)
 # ifdef MACOS_X_DARWIN
 #  include <mach/mach_host.h>
 #  include <mach/mach_port.h>
-#  include <mach/vm_page_size.h>
 # endif
 
 /*

src/version.c

@@ -766,6 +766,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1568,
 /**/
     1567,
 /**/