Update SConstruct to fix issue with EHXPLLL and related modules

iverilog fails compilation when working with ECP5 primitives such as EHXPLLL. Not exactly sure this is the correct fix, but linking "ecp5/cells_bb.v" (since that's where the EHXPLLL definition is) appears to allow compilation to proceed.

apio/resources/ecp5/SConstruct

@@ -203,7 +203,7 @@ def iverilog_generator(source, target, env, for_signature):
     #     end
     is_interactive_sim = is_testbench and 'sim' in COMMAND_LINE_TARGETS
     interactive_sim_flag = f'-D INTERACTIVE_SIM' if is_interactive_sim else ""
-    result = 'iverilog {0} -o $TARGET {1} {2} -D NO_INCLUDES "{3}/ecp5/cells_sim.v" $SOURCES'.format(
+    result = 'iverilog {0} -o $TARGET {1} {2} -D NO_INCLUDES "{3}/ecp5/cells_bb.v" "{3}/ecp5/cells_sim.v" $SOURCES'.format(
         IVER_PATH, vcd_output_flag, interactive_sim_flag, YOSYS_PATH)
     return result