powerpc/xive: Properly use static keyword for inline function

Fix fatal warning during compilation:

In file included from arch/powerpc/xmon/xmon.c:54:0:
./arch/powerpc/include/asm/xive.h:157:20: error: no previous prototype for ‘xive_smp_prepare_cpu’ [-Werror=missing-prototypes]
 extern inline int  xive_smp_prepare_cpu(unsigned int cpu) { return -EINVAL; }
                    ^

Signed-off-by: Mathieu Malaterre <malat@debian.org>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/xive.h

@@ -122,7 +122,7 @@ static inline bool xive_enabled(void) { return false; }
 static inline bool xive_spapr_init(void) { return false; }
 static inline bool xive_native_init(void) { return false; }
 static inline void xive_smp_probe(void) { }
-extern inline int  xive_smp_prepare_cpu(unsigned int cpu) { return -EINVAL; }
+static inline int  xive_smp_prepare_cpu(unsigned int cpu) { return -EINVAL; }
 static inline void xive_smp_setup_cpu(void) { }
 static inline void xive_smp_disable_cpu(void) { }
 static inline void xive_kexec_teardown_cpu(int secondary) { }