Add missing #include <unistd.h> (#975)

After the call to unlink() was added to util/io/file.cpp starting with
v2.2.3, the build fails on toolchains that do not transitively include
unistd.h, such as FreeBSD 15.1.  POSIX specifies that this function is
declared in unistd.h, so add the missing #include.

Author: Jehops

src/util/io/file.cpp

@@ -21,6 +21,8 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #ifdef _WIN32
 #include <io.h>
 #include <fcntl.h>
+#else
+#include <unistd.h> // unlink
 #endif
 #include <string.h> // strerror
 #include "file.h"