Ext4Pkg: Fix broken stack protector-enabled builds

As reported by Rebecca Cran, there was some build breakage when
compiling with stack-protector-on-by-default toolchains. Adding the
proper library to the DSC should fix it.

Cc: Rebecca Cran <rebecca@quicinc.com>
Cc: Marvin Häuser <mhaeuser@posteo.de>
Signed-off-by: Pedro Falcato <pedro.falcato@gmail.com>
Reviewed-by: Rebecca Cran <rebecca@quicinc.com>
Reviewed-by: Marvin Häuser <mhaeuser@posteo.de>