Makefile: Remove xelatex build log on clean

Previously the log files from xelatex were being left behind. Also, as
xelatex now builds the generated userguide.tex, the arguments to rm can
be matched by the userguide.* wildcard along with *.tex to remove the
remaining pandoc output.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
diff --git a/Makefile b/Makefile
index c00b79c..261867b 100644
--- a/Makefile
+++ b/Makefile
@@ -18,4 +18,4 @@
 	pandoc -o $@ $^
 
 clean:
-	rm -f *.aux *.tex *.out *.pdf
+	rm -f *.tex userguide.*