examples/embedding: Don't prefix $(MPTOP) to ports/unix source files.

Otherwise the build process puts the corresponding output object files in
two directories lower, not in build/ports/unix.

examples/embedding/Makefile.upylib

@@ -128,7 +128,7 @@ endif
 MAIN_C = main.c
 
 # source files
-SRC_C = $(addprefix $(MPTOP)/ports/unix/,\
+SRC_C = $(addprefix ports/unix/,\
 	$(MAIN_C) \
 	gccollect.c \
 	unix_mphal.c \