declare `char' fields used as signed

With some compilers or platforms, `char' means an unsigned
value, so we have to be explicit with `signed char'.

src/racket/src/future.h

@@ -184,7 +184,7 @@ typedef struct future_t {
   void *retval_p; /* use only with conservative GC */
   MZ_MARK_STACK_TYPE retval_m;
   int retval_i;
-  char no_retval;
+  signed char no_retval;
   char retval_is_rs_plus_two; /* => special result handling for on-demand JIT */
 
   Scheme_Object **multiple_array;

src/racket/src/schpriv.h

@@ -2956,7 +2956,7 @@ typedef struct Scheme_Module_Registry {
 struct Scheme_Env {
   Scheme_Object so; /* scheme_namespace_type */
 
-  char disallow_unbound, rename_set_ready;
+  signed char disallow_unbound, rename_set_ready;
 
   struct Scheme_Module *module; /* NULL => top-level */