Fix implicit conversion in workgen.cxx

[366/542] Building CXX object bench/workgen/CMakeFiles/workgen_cxx.dir/workgen.cxx.o

../bench/workgen/workgen.cxx:1197:33: warning: implicit conversion from 'unsigned int' to 'float' changes value from 4294967295 to 4294967296 [-Wimplicit-int-float-conversion]

    return (((float)r * sign) / UINT32_MAX);

                              ~ ^~~~~~~~~~

/usr/include/stdint.h:129:23: note: expanded from macro 'UINT32_MAX'

# define UINT32_MAX             (4294967295U)

                                 ^~~~~~~~~~~

1 warning generated. 

$ cmake -DCMAKE_TOOLCHAIN_FILE=../cmake/toolchains/clang.cmake -G Ninja ../

-- The C compiler identification is Clang 10.0.0

-- The CXX compiler identification is Clang 10.0.0 

(...)