tcg: Fix compilation without TCG

Commit 2726627197 started to use tb_unlock() and tlb_set_dirty() on
non TCG code.  Add the functions as stubs, so that builds with TCG
disabled continue to compile.

Signed-off-by: Juan Quintela <quintela@redhat.com>
Acked-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
[PMM: tweaked commit message]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

accel/stubs/tcg-stub.c

@@ -20,3 +20,11 @@
 void tb_flush(CPUState *cpu)
 {
 }
+
+void tb_unlock(void)
+{
+}
+
+void tlb_set_dirty(CPUState *cpu, target_ulong vaddr)
+{
+}