fix type of S_unwind_scan_frames

This function is registered with SAVEDESTRUCTOR_X, so its type must be
'void (pTHX_ void *)' exactly. (Also, there is no point in declaring p
pointer-to-const since we cast away 'const' three lines down anyway.)

I don't know if this change has any practical impact, but it shows up in
ASan output:

    scope.c:1543:13: runtime error: call to function S_unwind_scan_frames through pointer to incorrect function type 'void (*)(struct interpreter *, void *)'

Author: mauke

embed.fnc

@@ -5679,7 +5679,7 @@ ES	|void	|ssc_union	|NN regnode_ssc *ssc			\
 				|NN SV * const invlist			\
 				|const bool invert_2nd
 ES	|void	|unwind_scan_frames					\
-				|NN const void *p
+				|NN void *p
 #endif /* defined(PERL_IN_REGCOMP_STUDY_C) */
 #if defined(PERL_IN_REGEXEC_C)
 ERS	|LB_enum|advance_one_LB |NN U8 **curpos 			\

proto.h

@@ -25,7 +25,7 @@
 
 
 STATIC void
-S_unwind_scan_frames(pTHX_ const void *p)
+S_unwind_scan_frames(pTHX_ void *p)
 {
     PERL_ARGS_ASSERT_UNWIND_SCAN_FRAMES;
     scan_frame *f= (scan_frame *)p;