kgdb,x86: fix warning about unused variable

When compiling without CONFIG_DEBUG_RODATA the following
compiler warning is generated:

arch/x86/kernel/kgdb.c: In function 'kgdb_arch_set_breakpoint':
arch/x86/kernel/kgdb.c:749: warning: unused variable 'opc'

The variable instantiation needs to be inside the #ifdef to
make the warning go away.

Reported-by: Thiago Rafael Becker <trbecker@trbecker.org>
Signed-off-by: Jason Wessel <jason.wessel@windriver.com>