Remove bogus transitive includes.

<signal.h> shouldn't get you the contents of <errno.h>, and <fcntl.h>
shouldn't get you the contents of <unistd.h>.

Change-Id: I347499cd8671bfee98e6b8e875a97cab3a3655d3

libc/include/fcntl.h

@@ -35,7 +35,6 @@
 #include <linux/fcntl.h>
 #include <linux/stat.h>
 #include <linux/uio.h>
-#include <unistd.h>  /* this is not required, but makes client code much happier */
 
 __BEGIN_DECLS