use <poll.h> instead of <sys/poll.h>

The manpage of poll(2) states that the prototype of poll is defined
in <poll.h>. Use that header file instead of <sys/poll.h> to allow
compilation against musl-libc.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

disk/core.c

diff --git a/disk/core.c b/disk/core.c
index 309e16c..dd2f258 100644
--- a/disk/core.c
+++ b/disk/core.c
@@ -5,7 +5,7 @@
 
 #include <linux/err.h>
 #include <sys/eventfd.h>
-#include <sys/poll.h>
+#include <poll.h>
 
 #define AIO_MAX 256