meson: Don't build imgui by default
Otherwise imgui is built all the time, whether anything that uses it is built or not.
Fixes 51047cd2 ("build: move imgui out of src/intel/tools to be reused")
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Otherwise imgui is built all the time, whether anything that uses it is built or not.
Fixes 51047cd2 ("build: move imgui out of src/intel/tools to be reused")