Declare argument of nss_init const

nss_init() does not modify its path argument, thus declare it const.
Also drop superfluous prototype.

nss.c:54:31: warning: assignment discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
   54 |                 nsswitch_path = NSSWITCH;
      |                               ^

libmisc/idmapping.h

@@ -17,7 +17,5 @@ extern struct map_range *get_map_ranges(int ranges, int argc, char **argv);
 extern void write_mapping(int proc_dir_fd, int ranges,
 	struct map_range *mappings, const char *map_file, uid_t ruid);
 
-extern void nss_init(char *nsswitch_path);
-
 #endif /* _ID_MAPPING_H_ */