xpath BUGFIX unary expressions must be handled separately

Fixes #393
diff --git a/src/xpath.h b/src/xpath.h
index 86a4823..f83751a 100644
--- a/src/xpath.h
+++ b/src/xpath.h
@@ -118,6 +118,7 @@
     LYXP_EXPR_RELATIONAL,
     LYXP_EXPR_ADDITIVE,
     LYXP_EXPR_MULTIPLICATIVE,
+    LYXP_EXPR_UNARY,
     LYXP_EXPR_UNION,
 };