Move cpu_has_work and cpu_pc_from_tb to cpu.h

Move functions cpu_has_work() and cpu_pc_from_tb() from exec.h to cpu.h. This is
needed by later patches.

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
diff --git a/exec-all.h b/exec-all.h
index efef1cd..21a69d6 100644
--- a/exec-all.h
+++ b/exec-all.h
@@ -40,6 +40,7 @@
 #define DISAS_UPDATE  2 /* cpu state was modified dynamically */
 #define DISAS_TB_JUMP 3 /* only pc was modified statically */
 
+struct TranslationBlock;
 typedef struct TranslationBlock TranslationBlock;
 
 /* XXX: make safe guess about sizes */