Salut Nico,
Here are my two pence worth.
Your LINKER error means that you have included the new C-file properly in your test.c.
However, a symbol (= variable, array, constant... whatever) from uCOS is declared in its files but is not declared in test.c.
From the name of the symbol, I guess the function "OSInit" prototype need to be added as an extern in test.c for the LINKER to see the reference between the two files...
May be...
Bonne Chance,
Alban.