gallium: untrack u_indices_gen.c and u_indices_gen.c

gallium: untrack u_indices_gen.c and u_indices_gen.c

These files are built with make and removed with make clean, so it does not
seem necessary to track them.

Looking at the Makefile, it seems that the two u_indices_* files are handled
similarly to u_format_srgb.c u_format_table.c and u_half.c, and these 3
files are already untracked and in .gitignore

Signed-off-by: Xavier Chantry <chantry.xavier@gmail.com>
Signed-off-by: José Fonseca <jfonseca@vmware.com>