Apply a patch from Gisle Vanem to make tor-gencert build under MSVC

(Note: It makes sense to use tor-gencert on Windows for testing
purposes only.  If you are a directory authority operator, and you
are contemplating running tor-gencert on a Windows box in an actual
production environment, you are probably making a mistake.)

src/tools/tor-gencert.c

@@ -9,7 +9,9 @@
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <fcntl.h>
+#ifdef HAVE_UNISTD_H
 #include <unistd.h>
+#endif
 
 #include <openssl/evp.h>
 #include <openssl/pem.h>