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
diff --git a/libc/bionic/poll.cpp b/libc/bionic/poll.cpp
index ebb318d..d267229 100644
--- a/libc/bionic/poll.cpp
+++ b/libc/bionic/poll.cpp
@@ -26,6 +26,7 @@
  * SUCH DAMAGE.
  */
 
+#include <errno.h>
 #include <sys/poll.h>
 #include <sys/select.h>