Add some 'const' markers to static data that is really constant.

FossilOrigin-Name: e7073e23b80e7cae0c76d42f014ee9b2d40a8f2f
diff --git a/src/sqliteInt.h b/src/sqliteInt.h
index 2fa474c..eed2b10 100644
--- a/src/sqliteInt.h
+++ b/src/sqliteInt.h
@@ -2536,8 +2536,8 @@
 
 
 #ifndef SQLITE_MUTEX_OMIT
-  sqlite3_mutex_methods *sqlite3DefaultMutex(void);
-  sqlite3_mutex_methods *sqlite3NoopMutex(void);
+  sqlite3_mutex_methods const *sqlite3DefaultMutex(void);
+  sqlite3_mutex_methods const *sqlite3NoopMutex(void);
   sqlite3_mutex *sqlite3MutexAlloc(int);
   int sqlite3MutexInit(void);
   int sqlite3MutexEnd(void);