work around flex bug

flex emits code using isatty(), but fails to include the required
unistd.h. we can work around it by including the header ourselves.

Task-number: QTBUG-64771
Change-Id: I05313eeb79f7a0e25365dee5f05a0142f87209ae
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/tools/qlalr/examples/qparser/calc.l

@@ -33,6 +33,9 @@
 #include "calc_parser.h"
 #include <cstdlib>
 
+// Work around flex bug
+#include <unistd.h>
+
 #define YY_DECL int CalcParser::nextToken()
 %}