Avoid occasional confusion of Flymake C/C++ backend

The regexp would sometimes match non-errors and create a diagnostic
with nil severity, which would have consequences for the Flymake
engine (these should eventually be addressed, too).

* lisp/progmodes/flymake-cc.el (flymake-cc--make-diagnostics):
Tighten regexp.
1 job for master in 50 minutes and 40 seconds (queued for 3 seconds)
Status Job ID Name Coverage
failed #1120


Name Stage Failure
test-all Test
Makefile:313: recipe for target 'check-doit' failed
make[2]: *** [check-doit] Error 1
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:288: recipe for target 'check-expensive' failed
make[1]: *** [check-expensive] Error 2
make[1]: Leaving directory '/builds/emacs/emacs/test'
Makefile:962: recipe for target 'check-expensive' failed
make: *** [check-expensive] Error 2
ERROR: Job failed: exit code 1