os_support: Rename the poll fallback function to ff_poll

The fallback function is a non-static function, we shouldn't be
defining non-static functions outside of the proper ff/av prefix
namespaces.

This is especially important for a function like poll, which
other parties (other libraries, or executables linking these
libraries) also might provide similar but incompatible fallbacks for.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/os_support.c

@@ -266,7 +266,7 @@ int ff_socket_nonblock(int socket, int enable)
 }
 
 #if !HAVE_POLL_H
-int poll(struct pollfd *fds, nfds_t numfds, int timeout)
+int ff_poll(struct pollfd *fds, nfds_t numfds, int timeout)
 {
     fd_set read_set;
     fd_set write_set;

libavformat/os_support.h

@@ -101,7 +101,8 @@ struct pollfd {
 #endif
 
 
-int poll(struct pollfd *fds, nfds_t numfds, int timeout);
+int ff_poll(struct pollfd *fds, nfds_t numfds, int timeout);
+#define poll ff_poll
 #endif /* HAVE_POLL_H */
 #endif /* CONFIG_NETWORK */