A not X for was_lvalue_sub in embed.fnc.

7b70e81 was my (wrong) suggestion,
and it made the symbol only visible when PERL_CORE was defined,
which it isn't in List::Util.

embed.fnc

@@ -546,7 +546,7 @@ p	|bool	|io_close	|NN IO* io|bool not_implicit
 pR	|OP*	|invert		|NULLOK OP* cmd
 ApR	|I32	|is_lvalue_sub
 : Used in cop.h
-XpR	|I32	|was_lvalue_sub
+ApR	|I32	|was_lvalue_sub
 ApPR	|U32	|to_uni_upper_lc|U32 c
 ApPR	|U32	|to_uni_title_lc|U32 c
 ApPR	|U32	|to_uni_lower_lc|U32 c

embed.h

@@ -669,6 +669,7 @@
 #ifndef PERL_IMPLICIT_CONTEXT
 #define warner			Perl_warner
 #endif
+#define was_lvalue_sub()	Perl_was_lvalue_sub(aTHX)
 #define whichsig(a)		Perl_whichsig(aTHX_ a)
 #if !(defined(HAS_SIGACTION) && defined(SA_SIGINFO))
 #define csighandler		Perl_csighandler
@@ -1151,7 +1152,6 @@
 #define unshare_hek(a)		Perl_unshare_hek(aTHX_ a)
 #define vivify_ref(a,b)		Perl_vivify_ref(aTHX_ a,b)
 #define wait4pid(a,b,c)		Perl_wait4pid(aTHX_ a,b,c)
-#define was_lvalue_sub()	Perl_was_lvalue_sub(aTHX)
 #define watch(a)		Perl_watch(aTHX_ a)
 #define write_to_stderr(a)	Perl_write_to_stderr(aTHX_ a)
 #define yyerror(a)		Perl_yyerror(aTHX_ a)