Fix "make uninstall" for documentation

During "make uninstall", the documentatin was not deleted properly,
if DESTDIR was used.  The existing commands expanded to:

make[3]: Entering directory '/home/osmocom-build/jenkins/workspace/master-rtl-sdr/a1/default/a2/default/a3/default/label/osmocom-master-debian9/librtlsdr-0.5.4.5-5c30/_build/sub'
cd /tmp/am-dc-24621/ && rm -rf /home/osmocom-build/jenkins/workspace/master-rtl-sdr/a1/default/a2/default/a3/default/label/osmocom-master-debian9/librtlsdr-0.5.4.5-5c30/_inst/share/doc/librtlsdr/librtlsdr-0.5.4.5-5c30/html

This fixes the "make distcheck" step of rtl-sdr, and hence should also
fix the long-standing jenkins build verification for rtl-sdr
at https://jenkins.osmocom.org/jenkins/job/master-rtl-sdr/

Signed-off-by: Harald Welte <laforge@gnumonks.org>

Makefile.am

@@ -40,7 +40,7 @@ install-data-hook:
 	cd $(DESTDIR)$(doc_htmldir) && tar xf html.tar --strip-components 1 && rm -f html.tar
 
 uninstall-hook:
-	cd $(DESTDIR) && rm -rf $(doc_htmldir)
+	rm -rf $(DESTDIR)/$(doc_htmldir)
 
 DX_CLEAN = doc/{html,latex}/* doc/html.tar