fix group_wait test in cmake build.

[iskunk]

Ran into this error in a CMake build of the Git source with -DBUILD_TESTING=ON:

[ 46%] Building CXX object libs/process/test/CMakeFiles/boost_process_group_wait.dir/group_wait.cpp.o
In file included from /tmp/boost/libs/system/include/boost/system/error_category.hpp(11),
                 from /tmp/boost/libs/system/include/boost/system/error_code.hpp(14),
                 from /tmp/boost/libs/process/test/group_wait.cpp(15):
/tmp/boost/libs/system/include/boost/system/detail/error_category_impl.hpp(141): warning #2196: routine is both "inline" and "noinline"
  inline BOOST_NOINLINE error_category::operator std::error_category const & () const
         ^

/tmp/boost/libs/process/test/group_wait.cpp(19): catastrophic error: cannot open source file "/tmp/boost/libs/process/test/group_wait.cpp"
  #include <boost/scope_exit.hpp>
                                 ^

compilation aborted for /tmp/boost/libs/process/test/group_wait.cpp (code 4)
make[2]: *** [libs/process/test/CMakeFiles/boost_process_group_wait.dir/build.make:76: libs/process/test/CMakeFiles/boost_process_group_wait.dir/group_wait.cpp.o] Error 4
make[1]: *** [CMakeFiles/Makefile2:37090: libs/process/test/CMakeFiles/boost_process_group_wait.dir/all] Error 2
make: *** [Makefile:146: all] Error 2

Not sure I added Boost::scope_exit in the right place, but this at least got the build working again for me.