x86/orc: Make the is_callthunk() definition depend on CONFIG_BPF_JIT=y

Recent commit:

  02012623 Revert "x86/orc: Make it callthunk aware"

Made the only user of is_callthunk() depend on CONFIG_BPF_JIT=y, while
the definition of the helper function is unconditional.

Move is_callthunk() inside the #ifdef block.

Addresses this build failure:

   arch/x86/kernel/callthunks.c:296:13: error: ‘is_callthunk’ defined but not used [-Werror=unused-function]

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: linux-kernel@vger.kernel.org
Cc: Peter Zijlstra <peterz@infradead.org>