Add dependency on prim_clock_gating

Ibex depends on a clock gating primitive. This has always been the case;
previously, we have under-specified the dependency list by simply not
including this dependency. This is problematic not only because "IT'S
WRONG!", but also because it breaks self-contained targets, like
Verilator lint, which cannot run on ibex_core or ibex_core_tracing
without having a way to find the clock gating primitive.

ibex_core.core

@@ -9,6 +9,7 @@ filesets:
   files_rtl:
     depend:
       - lowrisc:prim:assert
+      - lowrisc:prim:clock_gating
       # TODO: Only lfsr is needed. Replace with a more specific dependency
       # once available.
       - lowrisc:prim:all