unzck: fix build with musl libc

unzck: fix build with musl libc

On musl libc "stdout" is a preprocessor macro whose expansion leads to
compilation errors.

Fixes:

| In file included from ../git/src/unzck.c:31:
| ../git/src/unzck.c: In function 'parse_opt':
| ../git/src/unzck.c:78:24: error: expected identifier before '(' token
|    78 |             arguments->stdout = true;
|       |                        ^~~~~~
| ../git/src/unzck.c: In function 'main':
| ../git/src/unzck.c:141:20: error: expected identifier before '(' token
|   141 |     if(!(arguments.stdout)) {
|       |                    ^~~~~~

Signed-off-by: Pierre-Jean Texier <pjtexier@koncepto.io>