Add a "fork" of musl as //fusl.

This is musl verbatim at d5f8394f6ea9549607567bd92de12a2446c15614.

See http://www.musl-libc.org/.

R=kulakowski@chromium.org

Review URL: https://codereview.chromium.org/1573973002 .
diff --git a/fusl/include/lastlog.h b/fusl/include/lastlog.h
new file mode 100644
index 0000000..5fa45ee
--- /dev/null
+++ b/fusl/include/lastlog.h
@@ -0,0 +1 @@
+#include <utmp.h>