fix build error 'strlen' is not a member of 'std' locally

Change-Id: I587ef2c38fd9377eef2073a49cd8497b2bd4bf9f
Reviewed-on: https://gerrit.libreoffice.org/10076Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
Tested-by: Stephan Bergmann <sbergman@redhat.com>

unoidl/source/sourcetreeprovider.cxx

@@ -11,6 +11,7 @@
 
 #include <map>
 #include <vector>
+#include <cstring>
 
 #include "osl/file.h"
 #include "osl/file.hxx"