test: undefined reference to 'sscanf'

Compiling with CONFIG_UNIT_TEST=y leads to:

aarch64-linux-gnu-ld.bfd:
test/lib/sscanf.c:50: undefined reference to `sscanf'

Add missing build dependency.

Fixes:e87dfb05 ("lib: sscanf: add sscanf implementation")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>