add sys/mman.h shim to define MAP_ANON if needed

thanks to kinichiro for pointing this out

ok deraadt@ beck@

include/sys/mman.h

@@ -0,0 +1,14 @@
+#include_next <sys/mman.h>
+
+#ifndef LIBCRYPTOCOMPAT_MMAN_H
+#define LIBCRYPTOCOMPAT_MMAN_H
+
+#ifndef MAP_ANONYMOUS
+#ifdef MAP_ANON
+#define MAP_ANONYMOUS MAP_ANON
+#else
+#error "System does not support mapping anonymous pages?"
+#endif
+#endif
+
+#endif