Removed header for label.h

Since the removal of the `label_table` global variables, the label
header does not need to be exposed publically to prevent accidental
change or corruption of these values.

libjas/include/jas.h

@@ -51,7 +51,6 @@ namespace jas {
 #include "codegen.h"
 #include "error.h"
 #include "instruction.h"
-#include "label.h"
 #include "mode.h"
 #include "operand.h"
 #include "register.h"