embed.fnc: Add 'const' to some prototypes

These are macros, so the const has no compilation effect, but the
function the macros expand to does have a const, so make them match.

embed.fnc

@@ -3080,22 +3080,22 @@ Adfpv	|void	|sv_catpvf_mg	|NN SV * const sv			\
 				|...
 Adp	|void	|sv_catpv_mg	|NN SV * const dsv			\
 				|NULLOK const char * const sstr
-AMbdp	|void	|sv_catpvn	|NN SV *dsv				\
+AMbdp	|void	|sv_catpvn	|NN SV * const dsv			\
 				|NN const char *sstr			\
 				|STRLEN len
 Adp	|void	|sv_catpvn_flags|NN SV * const dsv			\
 				|NN const char *sstr			\
 				|const STRLEN len			\
 				|const I32 flags
-AMbdp	|void	|sv_catpvn_mg	|NN SV *dsv				\
+AMbdp	|void	|sv_catpvn_mg	|NN SV * const dsv			\
 				|NN const char *sstr			\
 				|STRLEN len
 AMbdp	|void	|sv_catsv	|NN SV *dsv				\
 				|NULLOK SV *sstr
 Adp	|void	|sv_catsv_flags |NN SV * const dsv			\
 				|NULLOK SV * const sstr 		\
 				|const I32 flags
-AMbdp	|void	|sv_catsv_mg	|NN SV *dsv				\
+AMbdp	|void	|sv_catsv_mg	|NN SV * const dsv			\
 				|NULLOK SV *sstr
 Adp	|void	|sv_chop	|NN SV * const sv			\
 				|NULLOK const char * const ptr