Update serialization_lib submodule

New commits:
83c3a9c Fix warnings specific to GCC 9.4
43fdb52 Allow adding placeholders without data
3f7f423 Generate strict json by default when serializing

Signed-off-by: Ian Tayler Lessa <ian.taylerlessa@arm.com>
Change-Id: I8b62ccc73160688ec66e6f092ff972e418b9a231