ao_alsa: don't include alloca.h

It's true that ALSA uses alloca() in some of its API functions, but
since this is hidden behind macros in the ALSA headers, we have no
reason to include alloca.h ourselves.

Might help with portability (FreeBSD).

audio/out/ao_alsa.c

@@ -34,7 +34,6 @@
 #include <ctype.h>
 #include <math.h>
 #include <string.h>
-#include <alloca.h>
 
 #include "config.h"
 #include "mpvcore/options.h"

stream/ai_alsa1x.c

@@ -19,7 +19,6 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <sys/time.h>
-#include <alloca.h>
 
 #include "config.h"