[Examples] pytorch: Fix accessing /etc/fstab

This makes /etc/fstab accessible when running in SGX mode, which was
missed when we updated PyTorch recently.

Examples/pytorch/pytorch.manifest.template

@@ -170,6 +170,9 @@ sgx.allowed_files.hosts    = file:/etc/hosts
 sgx.allowed_files.gaiconf  = file:/etc/gai.conf
 sgx.allowed_files.resolv   = file:/etc/resolv.conf
 
+# System's file system table
+sgx.allowed_files.fstab   = file:/etc/fstab
+
 # Graphene optionally provides patched OpenMP runtime library that runs faster
 # inside SGX enclaves (execute `make -C LibOS gcc` to generate it). Uncomment
 # the lines below to use the patched library. PyTorch's SGX perf overhead