fix: include header for getcwd()

gcc complains about implicit declaration of getcwd, which is defined in
`unistd.h`.

See `man 3 getcwd`[0] for more information.

[0]: https://man.archlinux.org/man/getcwd.3

Author: grawlinson

src/mused.c

@@ -39,6 +39,7 @@ OTHER DEALINGS IN THE SOFTWARE.
 #include "diskop.h"
 #include <stdarg.h>
 #include <string.h>
+#include <unistd.h>
 
 extern Mused mused;
 extern Menu editormenu[];