Temporarily disable warnings for unknown pragmas (#942)

CUB 1.15 has uses an unprotected `#pragma nv_exec_check_disable` which makes includes of thrust headers from .cpp files fail if `-Werror` is enabled (or warn if it is not set)

This PR is a temporary workaround. It just adds -Wno-unknown-pragmas to the command line for building benchmark .cpp files.

Authors:
  - Mark Harris (https://github.com/harrism)

Approvers:
  - Robert Maynard (https://github.com/robertmaynard)
  - Bradley Dice (https://github.com/bdice)

URL: #942

benchmarks/CMakeLists.txt

@@ -45,8 +45,9 @@ function(ConfigureBench BENCH_NAME)
     target_compile_definitions(${TEST_NAME} PUBLIC "RMM_DISABLE_CUDA_MALLOC_ASYNC")
   endif()
 
-  target_compile_options(${BENCH_NAME} PUBLIC $<$<COMPILE_LANG_AND_ID:CXX,GNU,Clang>:-Wall -Werror
-                                              -Wno-error=deprecated-declarations>)
+  target_compile_options(
+    ${BENCH_NAME} PUBLIC $<$<COMPILE_LANG_AND_ID:CXX,GNU,Clang>:-Wall -Werror
+                         -Wno-error=deprecated-declarations -Wno-unknown-pragmas>)
   if(DISABLE_DEPRECATION_WARNING)
     target_compile_options(
       ${BENCH_NAME} PUBLIC $<$<COMPILE_LANGUAGE:CUDA>:-Xcompiler=-Wno-deprecated-declarations>)