Fix sqldiff.exe so that it always runs in single-thread mode.

FossilOrigin-Name: c223910e726131d7b718b556c83df5faa723369d
diff --git a/tool/sqldiff.c b/tool/sqldiff.c
index 0b3f02c..3aae46a 100644
--- a/tool/sqldiff.c
+++ b/tool/sqldiff.c
@@ -1139,6 +1139,7 @@
   char **azExt = 0;
 
   g.zArgv0 = argv[0];
+  sqlite3_config(SQLITE_CONFIG_SINGLETHREAD);
   for(i=1; i<argc; i++){
     const char *z = argv[i];
     if( z[0]=='-' ){