nx-X11/programs/Xserver/Imakefile: drop system libraries out of target list for nxagent regeneration.

Updating timestamps on object files and libraries used to compose
nxagent is only meaningful for actual files within the buildroot.
External dependencies would be ignored anyway, and worse, cause older
GNU Make versions to fail if they contain special characters like a
percent sign, which is typically interpreted as a wild card character.