imx-uuc: needs headers >= 3.18

The bump to 79e9798eee9cd972d92ed2b18f170de856d153c9 added the following
commit:
https://github.com/NXPmicro/imx-uuc/commit/ce808f3010ec73ccc07775c6cfd5317807a22415

This new ufb daemon uses usb_functionfs_descs_head_v2 which is available
only since 3.18:
https://github.com/torvalds/linux/commit/1df22b4ea9d91b01267fb61c155c31fb65d6b8a0

So add a dependency on headers >= 3.18

Fixes:
 - http://autobuild.buildroot.org/results/1081062b1c32c44a9feb5b14288095bc72d15986



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>