configure.ac: properly detect vasprintf

According POSIX the va_list is defined in stdarg.h
http://pubs.opengroup.org/onlinepubs/009695399/basedefs/stdarg.h.html

On some systems (like uClibc) the detection of vasprintf will fail
unless stdarg.h is included and build will later fail due to a vasprintf
redeclaration.
Signed-off-by: Natanael Copa <ncopa@alpinelinux.org>
Signed-off-by: Rafaël Carré <funman@videolan.org>

configure.ac

@@ -504,7 +504,8 @@ AC_LINK_IFELSE([
             c = NULL;
     ])],[AC_DEFINE([HAVE_ASPRINTF],[1],[Define to 1 if you have asprintf function])],[AC_LIBOBJ([asprintf])])
 AC_LINK_IFELSE([
-    AC_LANG_PROGRAM([#include <stdio.h>], [
+    AC_LANG_PROGRAM([#include <stdio.h>
+                     #include <stdarg.h>], [
         char *c;
         va_list ap;
         if (vasprintf(&c, "%s %d", ap) == -1)