kgdb: Provide a stub kgdb_nmicallback() if !CONFIG_KGDB

To save architectures from needing to wrap the call in #ifdefs, add a
stub no-op version of kgdb_nmicallback(), which returns 1 if it didn't
handle anything.
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20230601143109.v9.6.Ia3aeac89bb6751b682237e76e5ba594318e4b1aa@changeidSigned-off-by: Daniel Thompson <daniel.thompson@linaro.org>

include/linux/kgdb.h

@@ -365,5 +365,6 @@ extern void kgdb_free_init_mem(void);
 #define dbg_late_init()
 static inline void kgdb_panic(const char *msg) {}
 static inline void kgdb_free_init_mem(void) { }
+static inline int kgdb_nmicallback(int cpu, void *regs) { return 1; }
 #endif /* ! CONFIG_KGDB */
 #endif /* _KGDB_H_ */