Instantiate CUDA kernels for ConstantFlow

[mphoward]

Fixes a compilation issue with v0.9.0 reported by @ara137

[ 98%] Linking CXX executable example_test
../_azplugins.cpython-35m-x86_64-linux-gnu.so: undefined reference to 
`cudaError 
azplugins::gpu::brownian_flow<azplugins::ConstantFlow>(double4*, int3*, 
BoxDim const&, double4 const*, unsigned int const*, unsigned int const*, 
double const*, double, double const*, unsigned int, 
azplugins::ConstantFlow const&, unsigned int, double, double, unsigned 
int, unsigned int, bool, bool, unsigned int)'
../_azplugins.cpython-35m-x86_64-linux-gnu.so: undefined reference to 
`cudaError 
azplugins::gpu::langevin_flow_step2<azplugins::ConstantFlow>(double4*, 
double3*, double4 const*, double4 const*, unsigned int const*, unsigned 
int const*, double const*, double, double const*, unsigned int, 
azplugins::ConstantFlow const&, unsigned int, double, double, unsigned 
int, unsigned int, bool, bool, unsigned int)'
collect2: error: ld returned 1 exit status
azplugins/test/CMakeFiles/example_test.dir/build.make:101: recipe for 
target 'azplugins/test/example_test' failed
make[2]: *** [azplugins/test/example_test] Error 1
CMakeFiles/Makefile2:239: recipe for target 
'azplugins/test/CMakeFiles/example_test.dir/all' failed
make[1]: *** [azplugins/test/CMakeFiles/example_test.dir/all] Error 2
Makefile:138: recipe for target 'all' failed
make: *** [all] Error 2

This missing template causes a linker error, as it should. Weirdly, this is not an error in the Ubuntu 18 test container.