Add missing xpoly contact

Skywater's high precision resistors require dedicated contacts
and can't simply be contacted to with a simple poly contact.

While the DRC rules catch an attempt of contacting xpolyres with
a normal poly contact, the contact definitions didn't reflect that.

sky130/magic/sky130.tech

diff --git a/sky130/magic/sky130.tech b/sky130/magic/sky130.tech
index ac4e466..2fdf724 100644
--- a/sky130/magic/sky130.tech
+++ b/sky130/magic/sky130.tech
@@ -6860,6 +6860,7 @@
 #endif (METAL5)
 
  contact pc  170  poly 50 80 li 0 80
+ contact xpc 170 xpolyres 50 80 li 0 80
  contact pdc 170 pdiff 40 60 li 0 80
  contact ndc 170 ndiff 40 60 li 0 80
  contact psc 170   psd 40 60 li 0 80