docbook2mdoc: fix musl

srcpkgs/docbook2mdoc/patches/fix-decls.patch

@@ -0,0 +1,21 @@
+--- extern.h
++++ extern.h
+@@ -119,12 +119,16 @@ enum	attrval {
+ 	ATTRVAL__MAX
+ };
+ 
+-__BEGIN_DECLS
++#ifdef __cplusplus
++extern "C" {
++#endif
+ 
+ int isattrkey(enum nodeid node, enum attrkey key);
+ int isattrval(enum attrkey key, enum attrval val);
+ int isparent(enum nodeid node, enum nodeid parent);
+ 
+-__END_DECLS
++#ifdef __cplusplus
++}
++#endif
+ 
+ #endif