Add explicit (unsigned char*) cast in yxml_init()

Fixes #5.
diff --git a/yxml.c b/yxml.c
index 282b478..dbbc064 100644
--- a/yxml.c
+++ b/yxml.c
@@ -327,7 +327,7 @@
 void yxml_init(yxml_t *x, void *stack, size_t stacksize) {
 	memset(x, 0, sizeof(*x));
 	x->line = 1;
-	x->stack = stack;
+	x->stack = (unsigned char*)stack;
 	x->stacksize = stacksize;
 	*x->stack = 0;
 	x->elem = x->pi = x->attr = (char *)x->stack;
diff --git a/yxml.c.in b/yxml.c.in
index ab89b16..cd73e9e 100644
--- a/yxml.c.in
+++ b/yxml.c.in
@@ -261,7 +261,7 @@
 void yxml_init(yxml_t *x, void *stack, size_t stacksize) {
 	memset(x, 0, sizeof(*x));
 	x->line = 1;
-	x->stack = stack;
+	x->stack = (unsigned char*)stack;
 	x->stacksize = stacksize;
 	*x->stack = 0;
 	x->elem = x->pi = x->attr = (char *)x->stack;