add make clean to oss-fuzz build

Change-Id: Ic591748eedbe82b1c7305ef68156370f1b97f216
Reviewed-on: https://gerrit.libreoffice.org/58904Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

bin/oss-fuzz-build.sh

@@ -47,3 +47,5 @@ cp $SRC/*_seed_corpus.zip $OUT
 cp $SRC/*.dict $OUT
 #options files
 cp $SRC/libreoffice/vcl/workben/*.options $OUT
+
+make clean