objtool: Fix Clang enum conversion warning

Fix the following Clang enum conversion warning:

  arch/x86/decode.c:141:20: error: implicit conversion from enumeration
  type 'enum op_src_type' to different enumeration
  type 'enum op_dest_type' [-Werror,-Wenum-conversion]

    op->dest.type = OP_SRC_REG;
		  ~ ^~~~~~~~~~

It just happened to work before because OP_SRC_REG and OP_DEST_REG have
the same value.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Reviewed-by: Nicholas Mc Guire <der.herr@hofr.at>
Reviewed-by: Nick Desaulniers <nick.desaulniers@gmail.com>
Cc: Jiri Slaby <jslaby@suse.cz>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: baa41469

 ("objtool: Implement stack validation 2.0")
Link: http://lkml.kernel.org/r/b4156c5738bae781c392e7a3691aed4514ebbdf2.1514323568.git.jpoimboe@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>