fix missing perfcontext destroy declare in C API

Summary:
`rocksdb_perfcontext_destroy` declare is missing in C API.
Closes https://github.com/facebook/rocksdb/pull/3787

Differential Revision: D7816490

Pulled By: ajkr

fbshipit-source-id: 3a488607bfc897c7ce846a1b3c2b7af693134d0d

include/rocksdb/c.h

@@ -1076,6 +1076,8 @@ extern ROCKSDB_LIBRARY_API char* rocksdb_perfcontext_report(
     rocksdb_perfcontext_t* context, unsigned char exclude_zero_counters);
 extern ROCKSDB_LIBRARY_API uint64_t rocksdb_perfcontext_metric(
     rocksdb_perfcontext_t* context, int metric);
+extern ROCKSDB_LIBRARY_API void rocksdb_perfcontext_destroy(
+    rocksdb_perfcontext_t* context);
 
 /* Compaction Filter */