tests: Include poll.h rather than sys/poll.h

sys/poll.h is a non-standard location of the poll.h header, and is
incorrect on non-glibc libcs. poll.h, however, is defined in SUS (v2)
and is more portable.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=93764
http://pubs.opengroup.org/onlinepubs/007908799/xsh/poll.h.html
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>

tests/modetest/modetest.c

@@ -53,7 +53,7 @@
 #include <string.h>
 #include <strings.h>
 #include <errno.h>
-#include <sys/poll.h>
+#include <poll.h>
 #include <sys/time.h>
 #ifdef HAVE_SYS_SELECT_H
 #include <sys/select.h>

tests/vbltest/vbltest.c

@@ -35,7 +35,7 @@
 #include <unistd.h>
 #include <string.h>
 #include <errno.h>
-#include <sys/poll.h>
+#include <poll.h>
 #include <sys/time.h>
 #ifdef HAVE_SYS_SELECT_H
 #include <sys/select.h>