Add a dependency on regress-prep for sk-dummy.

This ensures the directory is created first and matches similar
dependencies for regress-binaries and regress-unit-binaries.  Spotted
by srxzr-ant Anthropic while onboarding OpenSSH to oss-scanner.

Makefile.in

@@ -768,7 +768,7 @@ SK_DUMMY_LIBRARY=@SK_DUMMY_LIBRARY@
 .c.lo: Makefile.in config.h
 	$(CC) $(CFLAGS_NOPIE) $(PICFLAG) $(CPPFLAGS) -c $< -o $@
 
-regress/misc/sk-dummy/sk-dummy.so: $(SK_DUMMY_OBJS)
+regress/misc/sk-dummy/sk-dummy.so: regress-prep $(SK_DUMMY_OBJS)
 	$(CC) $(CFLAGS) $(CPPFLAGS) $(PICFLAG) -shared -o $@ $(SK_DUMMY_OBJS) \
 	    -L. -Lopenbsd-compat -lopenbsd-compat $(LDFLAGS_NOPIE) $(TESTLIBS)