tools: Add missing unistd.h header

`close` was implicitly defined, include `unistd.h` to define it
explicitly so that the FreeBSD compile succeeds.

tools/headerversions.c

@@ -10,6 +10,7 @@
 #include <sqlite3.h>
 #include <sys/stat.h>
 #include <sys/types.h>
+#include <unistd.h>
 #include <zlib.h>
 
 static const char template[] =