Supply warn_on_slow_path() even for the !CONFIG_BUG case

Fix build problem with ACPI for !CONFIG_BUG. Noted by Randy Dunlap.

Signed-off-by: Andi Kleen <ak@linux.intel.com>

diff --git a/include/asm-generic/bug.h b/include/asm-generic/bug.h
index 2632328..d0d83b7 100644
--- a/include/asm-generic/bug.h
+++ b/include/asm-generic/bug.h
@@ -63,6 +63,9 @@ extern void warn_on_slowpath(const char *file, const int line);
 	unlikely(__ret_warn_on);					\
 })
 #endif
+
+static inline void warn_on_slowpath(const char *file, const int line) {}
+
 #endif
 
 #define WARN_ON_ONCE(condition)	({				\