Ensure $(DESTDIR)$(mono_libdir)/mono/3.5 exists before installing to it

Prior to 38348361dadd5d9e8a1721ecd3ea31b01952d1e7, the mono/X.X folders
were created when "gacutil /i foo.dll /package X.X" was called. However,
The above commit caused a call to $(INSTALL_DATA) to be executed *before*
gacutil, which caused a copy of Microsoft.Common.targets to be installed
as a file named mono/3.5 instead of into the mono/3.5 folder, on fresh
installs (not a problem installing into an existing prefix).

This commit forces creation of the folder which could potentially not
have been created yet by gacutil.