mxml-string: compile headers

This patch was taken from Debian's libmxml 2.11-1 source:
	04_compile_headers.diff

mxml-string.c

@@ -17,7 +17,8 @@
  */
 
 #include "config.h"
-
+#include <stdlib.h>
+#include <stdarg.h>
 
 /*
  * The va_copy macro is part of C99, but many compilers don't implement it.