Use poll.h instead of sys/poll.h

poll.h is the standard header name, and musl warns for including the wrong
one.

Signed-off-by: Bobby Bingham <koorogi@koorogi.info>
(cherry picked from commit 200e35e604)

flow/gsl/gslcommon.c

@@ -22,7 +22,7 @@
 #include <string.h>
 #include <sched.h>
 #include <errno.h>
-#include <sys/poll.h>
+#include <poll.h>
 #include <sys/stat.h>
 #include <sys/time.h>
 

flow/gsl/gslopmaster.c

@@ -17,7 +17,7 @@
  * Boston, MA 02110-1301, USA.
  */
 #include <string.h>
-#include <sys/poll.h>
+#include <poll.h>
 #include <sys/time.h>
 #include <errno.h>