x86/build: Add arch/x86/tools/insn_decoder_test to .gitignore

The file was generated by make command and should not be in the source tree.

Signed-off-by: Progyan Bhattacharya <progyanb@acm.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>