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: NThiago Rafael Becker <trbecker@trbecker.org>
Signed-off-by: NJason Wessel <jason.wessel@windriver.com>

arch/x86/kernel/kgdb.c

@@ -746,7 +746,9 @@ void kgdb_arch_set_pc(struct pt_regs *regs, unsigned long ip)
 int kgdb_arch_set_breakpoint(struct kgdb_bkpt *bpt)
 {
 	int err;
+#ifdef CONFIG_DEBUG_RODATA
 	char opc[BREAK_INSTR_SIZE];
+#endif /* CONFIG_DEBUG_RODATA */
 
 	bpt->type = BP_BREAKPOINT;
 	err = probe_kernel_read(bpt->saved_instr, (char *)bpt->bpt_addr,