xcursor: use memcpy() to append string of known size

Since len <= strlen(elt) is known, we don't need a str*() function. Let's
simply do memcpy() to suppress linter false positive.

Fixes #2777.

xcursor/xcursor.c

@@ -655,11 +655,8 @@ _XcursorAddPathElt (char *path, const char *elt, int len)
 	elt++;
 	len--;
     }
-#pragma GCC diagnostic push
+    memcpy (path + pathlen, elt, len);
-#pragma GCC diagnostic ignored "-Wstringop-truncation"
-    strncpy (path + pathlen, elt, len);
     path[pathlen + len] = '\0';
-#pragma GCC diagnostic pop
 }
 
 static char *