iproute2: fix build on glibc < 2.21 / uClibc

iproute2: fix build on glibc < 2.21 / uClibc

Fixes:
http://autobuild.buildroot.net/results/fc9/fc99b1ad49080f552ce611d5a7a2f9fe171b5a2c/
http://autobuild.buildroot.net/results/5f7/5f74c258a3adafa5aecd4abc378eef0573143764/

q_{etf,taprio}.c uses CLOCK_TAI, which isn't exposed by glibc < 2.21 or
uClibc, breaking the build. Provide a fallback definition like it is done
for IPPROTO_MPLS and others.

Patch submitted upstream:
https://lwn.net/ml/netdev/20181027153102.32302-1-peter%40korsgaard.com/

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