[OpenMP][Libomptarget] Fix cmake error on remote plugin

Requiring 3.15 causes a build breakage, I'm sure none of the contents actually require
3.15 or above.

Differential Revision: https://reviews.llvm.org/D95474

GitOrigin-RevId: 810572cc96e99b5d6fb695f1f047c2ae6507831b
1 file changed