Update smdlexp.vcxproj

Added a relative path to the required Module definition File smdlexp.def
found in halflife\utils\smdlexp

Although the project will compile fine without this step, 3DSMAX will
fail to load the plug-in giving errors such as "smdlexp.dle doesn't
implement LibVersion"

It seems that smdlexp.def needs to be explicitly defined in the vcxproj
in order to work.