Add all the glibc one-line "synonym" header files.

Ported code keeps needing these one by one, so let's just add the whole
set...

Change-Id: I9662b256aeaa1b0ffb22354130e503f0c4aa472b
diff --git a/libc/include/sys/unistd.h b/libc/include/sys/unistd.h
new file mode 100644
index 0000000..1e823fb
--- /dev/null
+++ b/libc/include/sys/unistd.h
@@ -0,0 +1 @@
+#include <unistd.h>