btrfs-progs: docs: add FS_VERITY_BUILTIN_SIGNATURES to fstests config

If not confiured, some tests fail with "kernel doesn't support fs-verity
builtin signatures".

[ci skip]

Signed-off-by: David Sterba <dsterba@suse.com>

Author: kdave

Documentation/dev/Development-notes.rst

@@ -516,6 +516,7 @@ Kernel config options for complete test coverage
 -  ``CONFIG_IO_URING=y``
 -  ``CONFIG_NUMA=y`` lack of *get_mempolicy()* syscall spills to test results
 -  ``CONFIG_USER_NS=y``
+-  ``CONFIG_FS_VERITY_BUILTIN_SIGNATURES=y``
 
 
 Kernel config options for better bug reports