S_new_SV: mark args unused, make inline, correct typo

When sv_inline.h was created in
Perl@75acd14
and a number of things moved into it, the S_new_SV debugging function should
have been made PERL_STATIC_INLINE. This commit now does that. It also marks
the arguments to S_new_SV as PERL_UNUSED_ARG, reducing warnings on some
builds.

sv_inline.h

@@ -71,10 +71,13 @@ SV* Perl_more_sv(pTHX);
 
 #ifdef DEBUG_LEAKING_SCALARS
 /* provide a real function for a debugger to play with */
-STATIC SV*
+PERL_STATIC_INLINE SV*
 S_new_SV(pTHX_ const char *file, int line, const char *func)
 {
     SV* sv;
+    PERL_UNUSED_ARG(file);
+    PERL_UNUSED_ARG(line);
+    PERL_UNUSED_ARG(func);
 
     if (PL_sv_root)
         uproot_SV(sv);