openvswitch: fix build for gcc versions below 4.9

Seems that while fixing the build for GCC 4.9
I broke the build for GCC 4.8, because that KCFLAG I added
causes a build error (since it's not recognized).

The fix is to add KCFLAG only when GCC 4.9 is used.

OpenVSwitch now builds successfully with GCC 4.8 and 4.9.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>