[libc++] Add missing FILE_DEPENDENCIES markup

Cr-Mirrored-From: https://chromium.googlesource.com/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: df88d80337d5e0ee0e50ca599d6436854951d8fa
diff --git a/test/std/input.output/file.streams/fstreams/filebuf.virtuals/pbackfail.pass.cpp b/test/std/input.output/file.streams/fstreams/filebuf.virtuals/pbackfail.pass.cpp
index 728eec2..03aff16 100644
--- a/test/std/input.output/file.streams/fstreams/filebuf.virtuals/pbackfail.pass.cpp
+++ b/test/std/input.output/file.streams/fstreams/filebuf.virtuals/pbackfail.pass.cpp
@@ -10,6 +10,8 @@
 
 // int_type pbackfail(int_type c = traits::eof());
 
+// FILE_DEPENDENCIES: underflow.dat
+
 #include <fstream>
 #include <cassert>