EntryCache: Add entry_cache_add_missing() for negative caching.

It provides a way for filesystems to cache a lookup failure and
therefore prevents repeated lookups of missing entries. This is a
common scenario for example in command lookup and compiling, where
each directory in PATH or each include directory is searched for the
given entry.

headers/private/fs_shell/fssh_api_wrapper.h

@@ -871,6 +871,7 @@
 
 /* entry cache */
 #define entry_cache_add					fssh_entry_cache_add
+#define entry_cache_add_missing			fssh_entry_cache_add_missing
 #define entry_cache_remove				fssh_entry_cache_remove
 
 ////////////////////////////////////////////////////////////////////////////////