replace sys/poll.h with poll.h

Fixes multiple warnings as this one:
```
/usr/include/sys/poll.h:1:2: warning: #warning redirecting incorrect #include <sys/poll.h> to <poll.h> [-Wcpp]
    1 | #warning redirecting incorrect #include <sys/poll.h> to <poll.h>
      |  ^~~~~~~
```

Acked-by: Yonggang Luo <luoyonggang@gmail.com>
Signed-off-by: David Heidelberg <david.heidelberg@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19587>

src/gallium/drivers/panfrost/pan_context.c

@@ -25,8 +25,8 @@
  *
  */
 
-#include <sys/poll.h>
 #include <errno.h>
+#include <poll.h>
 
 #include "pan_bo.h"
 #include "pan_context.h"

src/util/libsync.h

@@ -30,11 +30,11 @@
 
 #include <assert.h>
 #include <errno.h>
+#include <poll.h>
 #include <stdbool.h>
 #include <stdint.h>
 #include <string.h>
 #include <sys/ioctl.h>
-#include <sys/poll.h>
 #include <unistd.h>
 #include <time.h>