SPM: Fix missing break in TFM_SVC_OUTPUT_UNPRIV_STRING SVC handling

Missing break statement when handling TFM_SVC_OUTPUT_UNPRIV_STRING
in Library mode

Signed-off-by: Antonio de Angelis <antonio.deangelis@arm.com>
Change-Id: I13cee7e29a6879856f3a395ff36f223c994a395a