Fix XPath recursion limit

Fix accounting of recursion depth when parsing XPath expressions.

This silly bug introduced in commit 804c5297 could lead to spurious
errors when parsing larger expressions or XSLT documents.

Should fix #264.
diff --git a/xpath.c b/xpath.c
index 7497ba0..1aa2f1a 100644
--- a/xpath.c
+++ b/xpath.c
@@ -10983,7 +10983,7 @@
     }
 
     if (xpctxt != NULL)
-        xpctxt->depth -= 1;
+        xpctxt->depth -= 10;
 }
 
 /**