Include vm_page_size.h

The `vm_page_size` variable declared in `mach/vm_page_size.h` is used
when the header is available for inclusion. This header does not appear
to be included anywhere directly resulting in build failures due to the
missing `vm_page_size` declaration.

Author: etcwilde

Sources/_SubprocessCShims/process_shims.c

@@ -43,6 +43,10 @@
 extern char **environ;
 #endif
 
+#if __has_include(<mach/vm_page_size.h>)
+#include <mach/vm_page_size.h>
+#endif
+
 int _was_process_exited(int status) {
     return WIFEXITED(status);
 }