spirv-opt is not present when installing qtshadertools-tools package

When installing qtshadertools-tools it contains the `qsb` binary. It may use the `spirv-opt` if `qsb` is given the -O option. The current way of depending on the dependency is to manually specify it in our recipe. But we already depends on qtshadertools-native through qt3d.

It could be nice if it was available through qtshadertools as a packageconfig or other means.

When `spirv-opt` is not present we get the current warning/error from qsb.

Failed to run spirv-opt -O /tmp/qsb-XjLsUS/qsb_spv_temp -o /tmp/qsb-XjLsUS/qsb_spv_temp_out: execvp: No such file or directory

{}It should be available through the nativesdk too.