cmake: let libglobal_obj depend on legacy-option-headers

cmake: let libglobal_obj depend on legacy-option-headers

to address following build failure:

FAILED: src/global/CMakeFiles/libglobal_objs.dir/global_init.cc.o ...
src/global/CMakeFiles/libglobal_objs.dir/global_init.cc.o -MF src/global/CMakeFiles/libglobal_objs.dir/global_init.cc.o.d -o src/global/CMakeFiles/libglobal_objs.dir/global_init.cc.o -c
../src/global/global_init.cc
In file included from ../src/global/global_init.cc:26:
In file included from ../src/common/config.h:26:
In file included from ../src/common/config_values.h:59:
../src/common/options/legacy_config_opts.h:1:10: fatal error: 'global_legacy_options.h' file not found
         ^~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.

Signed-off-by: Kefu Chai <kchai@redhat.com>