Enable UMF_BUILD_BENCHMARKS_MT=ON in GPU CI flows

Author: vinser52

.github/workflows/reusable_gpu.yml

@@ -88,6 +88,7 @@ jobs:
           -DCMAKE_CXX_COMPILER=${{matrix.compiler.cxx}}
           -DUMF_BUILD_SHARED_LIBRARY=${{matrix.shared_library}}
           -DUMF_BUILD_BENCHMARKS=ON
+          -DUMF_BUILD_BENCHMARKS_MT=ON
           -DUMF_BUILD_TESTS=ON
           -DUMF_BUILD_GPU_TESTS=ON
           -DUMF_BUILD_GPU_EXAMPLES=ON