x86: Fix LOCK_PREFIX_HERE for uniprocessor build

Checkin b3ac891b:
x86: Add support for lock prefix in alternatives

... did not define LOCK_PREFIX_HERE in the case of a uniprocessor
build.  As a result, it would cause any of the usages of this macro to
fail on a uniprocessor build.  Fix this by defining LOCK_PREFIX_HERE
as a null string.
Signed-off-by: NH. Peter Anvin <hpa@zytor.com>
Cc: Luca Barbieri <luca@luca-barbieri.com>
LKML-Reference: <1267005265-27958-2-git-send-email-luca@luca-barbieri.com>

arch/x86/include/asm/alternative.h

@@ -38,6 +38,7 @@
 #define LOCK_PREFIX LOCK_PREFIX_HERE "\n\tlock; "
 
 #else /* ! CONFIG_SMP */
+#define LOCK_PREFIX_HERE ""
 #define LOCK_PREFIX ""
 #endif