add xschem package

[proppy]

• Upstream: https://github.com/StefanSchippers/xschem
• License: GPLv2+
• Recipe: https://xschem.sourceforge.io/stefan/xschem_man/install_xschem.html (standard autotools)
• Deps: x11, cairo, tcl/tk, xpm
• Build-deps: bison, flex

[proppy]

once xschem is packaged, it would make sense to enable the xschem symbol in https://github.com/hdl/conda-eda/blob/master/misc/open_pdks/build.sh#L60