docs: Try harder to uninstall

Imagine you have partially installed libvirt, or maybe you're
just running 'make uninstall' from a different version than 'make
install' has been ran. One way or another, we are doing plain
'rm' instead of 'rm -f' and thus not trying hard enough when
uninstalling. In the rest of our code we stick with -f switch. Do
that for docs too.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

docs/Makefile.am

@@ -366,8 +366,8 @@ install-data-local:
 	$(INSTALL_DATA) $(srcdir)/libvirtLogo.png $(DESTDIR)$(pkgdatadir)
 
 uninstall-local:
-	for h in $(apihtml); do rm $(DESTDIR)$(HTML_DIR)/$$h; done
-	for p in $(apipng); do rm $(DESTDIR)$(HTML_DIR)/$$p; done
+	for h in $(apihtml); do rm -f $(DESTDIR)$(HTML_DIR)/$$h; done
+	for p in $(apipng); do rm -f $(DESTDIR)$(HTML_DIR)/$$p; done
 	for f in $(devhelphtml) $(devhelppng) $(devhelpcss); do \
-	  rm $(DESTDIR)$(DEVHELP_DIR)/$$(basename $$f); \
+	  rm -f $(DESTDIR)$(DEVHELP_DIR)/$$(basename $$f); \
 	done