Adds function declarations

[jrincayc]

Newer C compilers are getting stricter about requiring function prototypes (GCC for example makes it an error in version 14 [https://gcc.gnu.org/gcc-14/porting_to.html] )

This pull request adds all needed function prototypes for compiling.

[jrincayc]

Note that -Wno-implicit-int is needed with GCC 14 to compile.

Also, an alternative to this is to compile with -Wno-implicit-function-declaration in GCC 14

[jrincayc]

If you would prefer K&R syntax, this can be changed to:

int AddList();
int GetList();
int GetObject();
void PrintList();
void PrintObject();
void PrintChar();
int Car();
int Cdr();
int Cons();
int Eval();

I would be happy to create the pull request that way instead if you would prefer.

(branch https://github.com/jrincayc/sectorlisp/tree/kandr )

[jrincayc]

Or you could completely eliminate implicit ints: https://github.com/jrincayc/sectorlisp/tree/no_implicit_int

[jart]

I've merged #33 in favor of this change, for reasons which I'll explain in greater detail in #34.