build: trivial fix error: implicit declaration of function 'malloc'

Fixes this error when building with -Werror on Alpine Linux:

util/processinfo.c: In function 'virProcessInfoSetAffinity':
util/processinfo.c:52:5: error: implicit declaration of function 'malloc' [-Werror=implicit-function-declaration]
Signed-off-by: NNatanael Copa <ncopa@alpinelinux.org>

src/util/processinfo.c

@@ -21,6 +21,7 @@
 
 #include <config.h>
 
+#include <stdlib.h>
 #include <sched.h>
 
 #include "processinfo.h"