Fix build with "-fno-common", from FreeBSD.
https://gitlab.com/mlmmj/mlmmj/-/merge_requests/2

Index: include/mlmmj.h
--- include/mlmmj.h.orig
+++ include/mlmmj.h
@@ -81,7 +81,7 @@ enum subtype {
 	SUB_NONE /* For when an address is not subscribed at all */
 };
 
-char *subtype_strs[7]; /* count matches enum above; defined in subscriberfuncs.c */
+extern char *subtype_strs[7]; /* count matches enum above; defined in subscriberfuncs.c */
 
 enum subreason {
 	SUB_REQUEST,
@@ -92,7 +92,7 @@ enum subreason {
 	SUB_SWITCH
 };
 
-char * subreason_strs[6]; /* count matches enum above; defined in subscriberfuncs.c */
+extern char * subreason_strs[6]; /* count matches enum above; defined in subscriberfuncs.c */
 
 void print_version(const char *prg);