new compilation warning in DepTable.cpp

[Uwe Stöhr]

With current master I get:

D:\LyXGit\Master\src\DepTable.cpp(46): warning C4244: '=': conversion 
from 'time_t' to 'long', possible loss of data 
[D:\LyXGit\Master\compile-2015\src\LyX.vcxproj]

JMarc, could you please have a look?

thanks and regards
Uwe