remote: Mark `git_remote_name_is_valid` as `GIT_EXTERN`

This change makes `git_remote_name_is_valid` be part of the public
interface of the library. This is needed for other language bindings to
be able to find this symbol (like in git2go, when linking against
libgit2 dynamically).

include/git2/remote.h

@@ -971,7 +971,7 @@ GIT_EXTERN(int) git_remote_rename(
  * @param remote_name name to be checked.
  * @return 0 on success or an error code
  */
-int git_remote_name_is_valid(int *valid, const char *remote_name);
+GIT_EXTERN(int) git_remote_name_is_valid(int *valid, const char *remote_name);
 
 /**
 * Delete an existing persisted remote.