backend compilation cache should not depend on CUDA

This fixes the WITH_CUDA=0 build broken by 4f90354 (Split backend
compilation cache, Sun Apr 8 13:12:14 2018 -0600).

Author: Sven Verdoolaege

tc/core/compilation_cache.cc

@@ -13,7 +13,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-#include "tc/core/cuda/cuda_compilation_cache.h"
+#include "tc/core/compilation_cache.h"
 
 #include <version.h>
 

tc/core/compilation_cache.h

@@ -26,9 +26,6 @@
 
 #include <compcache.pb.h>
 
-#include "tc/core/cuda/cuda.h"
-#include "tc/core/cuda/cuda_mapping_options.h"
-#include "tc/core/cuda/cuda_rtc.h"
 #include "tc/core/utils/time.h"
 
 namespace tc {