proc.h: stop including khash.h which is no longer used

Instead, we need to include <string.h> explicitly.

include/mruby/proc.h

@@ -9,6 +9,7 @@
 
 #include "common.h"
 #include <mruby/irep.h>
+#include <string.h>
 
 /**
  * Proc class
@@ -134,9 +135,6 @@ MRB_API mrb_value mrb_proc_cfunc_env_get(mrb_state *mrb, mrb_int idx);
 #define MRB_METHOD_CFUNC_P(m) (MRB_METHOD_FUNC_P(m)?TRUE:(MRB_METHOD_PROC(m)?(MRB_PROC_CFUNC_P(MRB_METHOD_PROC(m))):FALSE))
 #define MRB_METHOD_CFUNC(m) (MRB_METHOD_FUNC_P(m)?MRB_METHOD_FUNC(m):((MRB_METHOD_PROC(m)&&MRB_PROC_CFUNC_P(MRB_METHOD_PROC(m)))?MRB_PROC_CFUNC(MRB_METHOD_PROC(m)):NULL))
 
-
-#include <mruby/khash.h>
-
 MRB_API mrb_value mrb_load_proc(mrb_state *mrb, const struct RProc *proc);
 
 /**