doc make: Add *.pdf files to the 'clean' target

Signed-off-by: Vadim Kochan <vadim4j@gmail.com>
diff --git a/doc/Makefile b/doc/Makefile
index b92957e..e9c0ff7 100644
--- a/doc/Makefile
+++ b/doc/Makefile
@@ -70,4 +70,4 @@
 	install -m 0644 $(shell echo *.sgml) $(DESTDIR)$(DOCDIR)
 
 clean:
-	rm -f *.aux *.log *.toc $(PSFILES) $(DVIFILES) *.html
+	rm -f *.aux *.log *.toc $(PSFILES) $(DVIFILES) *.html *.pdf