[OpenMP][Libomptarget] Fix conditional in CMake for remote plugin

The remote offloading plugin's CMakeLists was trying to build if its
flag was enabled even if it didn't find gRPC/protobuf. The conditional
was wrong, it's fixed by this.

Differential Revision: https://reviews.llvm.org/D95574

GitOrigin-RevId: 8a77056256d9970387595a5c729d894e3fe07131
1 file changed