Fix builtin alsa headers building on musl

For some reason it doesn't use the POSIX `poll.h`, instead resorting to
`sys/poll.h`. Musl doesn't really like this at all, throwing a warning
and, thus, halting compilation.

thirdparty/linuxbsd_headers/alsa/asoundlib.h

@@ -35,7 +35,7 @@
 #include <string.h>
 #include <fcntl.h>
 #include <assert.h>
-#include <sys/poll.h>
+#include <poll.h>
 #include <errno.h>
 #include <stdarg.h>
 #include <endian.h>