codec_dahdi: Fix poll.h include.

POSIX defines poll.h. sys/poll.h should not be used as it is c-library
internal header which may or may not exist. Notably in musl including
sys/poll.h generates warning of being incorrect.

Change-Id: Ib318c1c7142a737bcf3caa4d8d72560bebe39252

codecs/codec_dahdi.c

@@ -34,11 +34,11 @@
 #include "asterisk.h"
 #include <stdbool.h>
 
+#include <poll.h>
 #include <fcntl.h>
 #include <netinet/in.h>
 #include <sys/ioctl.h>
 #include <sys/mman.h>
-#include <sys/poll.h>
 #include <dahdi/user.h>
 
 #include "asterisk/lock.h"