CUDA: Enable CudaOnly.SeparateCompilation test runtime calls

Uncomment the calls in `main.cpp` (and fix the signatures).

Tests/CudaOnly/SeparateCompilation/main.cu

@@ -4,12 +4,12 @@
 #include "file1.h"
 #include "file2.h"
 
-// result_type file4_launch_kernel(int x);
-// result_type file5_launch_kernel(int x);
+int file4_launch_kernel(int x);
+int file5_launch_kernel(int x);
 
 int main(int argc, char **argv)
 {
-  // file4_launch_kernel(42);
-  // file5_launch_kernel(42);
+  file4_launch_kernel(42);
+  file5_launch_kernel(42);
   return 0;
 }