dissect: Allow to show all symbols

Currently dissect sees only used symbols. For indexing purposes, it is
useful to see all declared symbols.

$ nl -s\  -w2 ./z.c
 1 struct foo {
 2         int member;
 3 };
 4 #ifdef OPT
 5 static void func1(void) {
 6         struct foo *x;
 7         return 0;
 8 }
 9 #endif
10 static inline void func2(void) { return; }
11 void func(void) { return; }

$ ./test-dissect ./z.c

FILE: ./z.c

  11:6                    def   f func                             void ( ... )

$ ./test-dissect --param=dissect-show-all-symbols ./z.c

FILE: ./z.c

   1:8                    def   s foo                              struct foo
   2:13                   def   m foo.member                       int
  10:20                   def   f func2                            void ( ... )
  11:6                    def   f func                             void ( ... )

Signed-off-by: Alexey Gladkov <gladkov.alexey@gmail.com>
Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>