liburcu: needs MMU
The test_urcu_fork.c test calls fork(). Upstream provides no easy way to
disable tests build.
Fixes:
http://autobuild.buildroot.net/results/9eb/9eb182ab2d0b396eeff3d637a040da5a4d5bdcd7/
http://autobuild.buildroot.net/results/47a/47aabc219a2fd50105881b1e2715f7eeb1a6a588/
http://autobuild.buildroot.net/results/566/56631605f76b8623c9674f25c529ac67d0de12a8/
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>