Include <unistd.h> for the unlink function.  This avoids an implicit
function declaration, increasing C99 compatibility.

Submitted upstream: <https://sourceforge.net/p/byaccj/patches/5/>

diff --git a/src/main.c b/src/main.c
index 54b5e05ba3aa2946..e4deacd47a3c0d34 100755
--- a/src/main.c
+++ b/src/main.c
@@ -4,6 +4,7 @@
 
 #ifndef __WIN32__  /*rwj  --  make portable*/
 #include <signal.h>
+#include <unistd.h>
 #else
 #include <dir.h>
 #endif