actually expose rocksdb_status_ptr_get_error via c api (facebook#13875)

Summary:
the function implementation is here: https://github.com/facebook/rocksdb/blob/8f0ab1598effd4b05f6f88310c7bd9aaf5d418c6/db/c.cc#L928-L930 but it wasn't fully exposed

Pull Request resolved: facebook#13875

Reviewed By: hx235

Differential Revision: D80717828

Pulled By: cbi42

fbshipit-source-id: d6aaa984f24e469aa8ddb81524dc156b85e891f2

Author: zaidoon1

include/rocksdb/c.h

@@ -1175,6 +1175,8 @@ extern ROCKSDB_LIBRARY_API void rocksdb_reset_status(
     rocksdb_status_ptr_t* status_ptr);
 extern ROCKSDB_LIBRARY_API uint32_t
 rocksdb_flushjobinfo_flush_reason(const rocksdb_flushjobinfo_t* info);
+extern ROCKSDB_LIBRARY_API void rocksdb_status_ptr_get_error(
+    rocksdb_status_ptr_t* status, char** errptr);
 
 /* Compaction job info */
 extern ROCKSDB_LIBRARY_API void rocksdb_compactionjobinfo_status(