tcg/ppc: Fix build of tcg_qemu_tb_exec()

Commit 0b0d3320db (TCG: Final globals
clean-up) moved code_gen_prologue but forgot to update ppc code.
This broke the build on 32-bit ppc. ppc64 is unaffected.

Cc: Evgeny Voevodin <evgenyvoevodin@gmail.com>
Cc: Blue Swirl <blauwirbel@gmail.com>
Signed-off-by: Andreas Färber <andreas.faerber@web.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>

tcg/ppc/tcg-target.h

@@ -99,6 +99,6 @@ typedef enum {
 
 #define tcg_qemu_tb_exec(env, tb_ptr) \
     ((long __attribute__ ((longcall)) \
-      (*)(void *, void *))code_gen_prologue)(env, tb_ptr)
+      (*)(void *, void *))tcg_ctx.code_gen_prologue)(env, tb_ptr)
 
 #endif