compiler: use *char() cast in C functions to afix warnings

vlib/builtin/array.v

@@ -276,7 +276,7 @@ pub fn (b []byte) hex() string {
 	mut hex := malloc(b.len*2+1)
 	mut ptr := &hex[0]
 	for i := 0; i < b.len ; i++ {
-		ptr += C.sprintf(ptr, '%02x', b[i])
+		ptr += C.sprintf(*char(ptr), '%02x', b[i])
 	}
 	return string(hex)
 }