libcamera: event_dispatcher_poll: Remove struct keyword from for-range

When using -std=c++11, GCC versions 6.2 and 6.3 take objection to
declaring a struct type when using a range based iterator:

    event_dispatcher_poll.cpp:231:13: error: types may not be defined
	in a for-range-declaration [-Werror]

		for (const struct pollfd &pfd : pollfds) {
		           ^~~~~~

	cc1plus: all warnings being treated as errors

Removing the keyword 'struct' ensures that the compiler does not try to
declare the type, and instead uses the type as already defined by the
relevant poll.h header.

This issue does not affect later compiler versions.

Reported-by: [autobuild.buildroot.net] Thomas Petazzoni <thomas.petazzoni@bootlin.com>
  http://autobuild.buildroot.net/results/f6dd4c60c04892c8b1669e6000fce7edb2b6349e/

Fixes: 8356f8a6ab ("libcamera: Add a poll-based event dispatcher")
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

src/libcamera/event_dispatcher_poll.cpp

@@ -241,7 +241,7 @@ void EventDispatcherPoll::processNotifiers(const std::vector<struct pollfd> &pol
 		{ EventNotifier::Exception, POLLPRI },
 	};
 
-	for (const struct pollfd &pfd : pollfds) {
+	for (const pollfd &pfd : pollfds) {
 		auto iter = notifiers_.find(pfd.fd);
 		ASSERT(iter != notifiers_.end());