Fix builds with both SQLITE_ENABLE_MULTITHREADED_CHECKS and
SQLITE_THREADSAFE=0 defined.

FossilOrigin-Name: 7d0b12fcb58353b883ffce77df824a5cc8b1e913a21ec3f22fb73481a398c916
diff --git a/src/sqliteInt.h b/src/sqliteInt.h
index c416754..27b896d 100644
--- a/src/sqliteInt.h
+++ b/src/sqliteInt.h
@@ -3590,7 +3590,7 @@
 sqlite3_mutex *sqlite3Pcache1Mutex(void);
 sqlite3_mutex *sqlite3MallocMutex(void);
 
-#ifdef SQLITE_ENABLE_MULTITHREADED_CHECKS
+#if defined(SQLITE_ENABLE_MULTITHREADED_CHECKS) && !defined(SQLITE_MUTEX_OMIT)
 void sqlite3MutexWarnOnContention(sqlite3_mutex*);
 #else
 # define sqlite3MutexWarnOnContention(x)