libv4l: fix utilities build with older glibc versions

libv4l: fix utilities build with older glibc versions

Fixes:
http://autobuild.buildroot.net/results/9af/9af37fbf344b63b60e59ccac21e010cdf0ea219e/
http://autobuild.buildroot.net/results/bbd/bbd34d5378354e9db7822eaae8d3c4a75a67ac23/
http://autobuild.buildroot.net/results/ebb/ebbaa70e1a277162ba53c7bde8335ee998486703/
http://autobuild.buildroot.net/results/2c1/2c11f4dbb7cfadcfc6fbc267679b2b4eab867221/

And many more.

The utilities use clock_gettime(), which was provided in librt in glibc <
2.17, so ensure we link against -lrt.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>