__BEGIN_DECLS / __END_DECLS are a glibc-only wrapper mechanism to make
C declarations work in C++ code. As musl does not support these and the
OpenSMTPD codebase is solely C anyway, simply remove them.

--- a/extras/tables/table-ldap/ber.h
+++ b/extras/tables/table-ldap/ber.h
@@ -75,7 +75,6 @@
 	size_t		bo_n;
 };
 
-__BEGIN_DECLS
 struct ber_element	*ber_get_element(unsigned long);
 void			 ber_set_header(struct ber_element *, int,
 			    unsigned long);
@@ -123,4 +122,3 @@
 void			 ber_set_application(struct ber *,
 			    unsigned long (*)(struct ber_element *));
 void			 ber_free(struct ber *);
-__END_DECLS