altp2m: address Misra 2.1 rule violation

The stub altp2m_vcpu_idx() is recognized as "noreturn" function lacking
respective annotation (or having a return statement), which hence is deemed
unreachable code by Misra / Eclair. All call sites are guarded by
altp2m_active() checks, hence an inline function isn't needed. A
declaration will suffice, with call sites then getting DCE-d.

No functional change.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

xen/include/asm-generic/altp2m.h

@@ -14,13 +14,8 @@ static inline bool altp2m_active(const struct domain *d)
     return false;
 }
 
-/* Alternate p2m VCPU */
-static inline unsigned int altp2m_vcpu_idx(const struct vcpu *v)
-{
-    /* Not implemented on GENERIC, should not be reached. */
-    BUG();
-    return 0;
-}
+/* Alternate p2m VCPU - placeholder on GENERIC */
+unsigned int altp2m_vcpu_idx(const struct vcpu *v);
 
 #endif /* __ASM_GENERIC_ALTP2M_H */