s390/disassembler: add generated gen_opcode_table tool to .gitignore

Fixes: 8bc1e4ec ("s390/disassembler: generate opcode tables from text file")
Signed-off-by: NHendrik Brueckner <brueckner@linux.vnet.ibm.com>

arch/s390/tools/.gitignore

 gen_facilities
+gen_opcode_table