shell: grant access to read /proc/filesystems

Addresses the following test failure:
system/extras/tests/kernel.config/nfs_test.cpp:24: Failure
Value of: android::base::ReadFileToString("/proc/filesystems", &fs)
Actual: false
Expected: true

Denial:
avc: denied { read } for name="filesystems" dev="proc"
scontext=u:r:shell:s0 tcontext=u:object_r:proc_filesystems:s0
tclass=file

Bug: 67862327
Test: build
Change-Id: I9ada5404987cb474968afc8cb8d96137ee36c68d

public/shell.te

@@ -109,6 +109,7 @@ allow shell hwservicemanager:hwservice_manager list;
 # allow shell to look through /proc/ for ps, top, netstat
 r_dir_file(shell, proc)
 r_dir_file(shell, proc_net)
+allow shell proc_filesystems:file r_file_perms;
 allow shell proc_interrupts:file r_file_perms;
 allow shell proc_meminfo:file r_file_perms;
 allow shell proc_stat:file r_file_perms;