systemc: Make input.txt a dependency for the tlm/endian_conv test.

This input file is consumed by the test and needs to be in the build
directory.

Change-Id: I3420dec9e41a1981c7d4e6df47f03273e378ab66
Reviewed-on: https://gem5-review.googlesource.com/c/15064
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Maintainer: Anthony Gutierrez <anthony.gutierrez@amd.com>