test: Fix declaration of main method

[nmeum]

After reading cabos comment in #44 I wondered if cn-cbor actually compiles with C compilers other than gcc and clang and tested this by compiling it with pcc using the Simple-Makefile.

pcc complained that the function declaration for main in test.c is incorrect and according to section 5.1.2.2.1 of the C99 standard pcc is actually right about this.

[cabo]

Not sure the test.c is particularly portable, as evidenced by

  system("leaks test");

(and the lack of a return...), but this should be an innocuous change.
Further portability fixes are welcome!

[nmeum]

(and the lack of a return...)

Section 5.1.2.2.3 of the C99 standard states the following:

[...] reaching the } that terminates the main function returns a value of 0.

So you probably don't need a return statement in that function.