tests/guest-debug: fix scoping of failcount

You should declare you are using a global version of a variable before
you attempt to modify it in a function.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20181109152119.9242-5-alex.bennee@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

tests/guest-debug/test-gdbstub.py

@@ -16,6 +16,7 @@ def report(cond, msg):
         print ("PASS: %s" % (msg))
     else:
         print ("FAIL: %s" % (msg))
+        global failcount
         failcount += 1