x86/mem_encrypt: Unbreak the AMD_MEM_ENCRYPT=n build

Moving mem_encrypt_init() broke the AMD_MEM_ENCRYPT=n because the
declaration of that function was under #ifdef CONFIG_AMD_MEM_ENCRYPT and
the obvious placement for the inline stub was the #else path.

This is a leftover of commit 20f07a04 ("x86/sev: Move common memory
encryption code to mem_encrypt.c") which made mem_encrypt_init() depend on
X86_MEM_ENCRYPT without moving the prototype. That did not fail back then
because there was no stub inline as the core init code had a weak function.

Move both the declaration and the stub out of the CONFIG_AMD_MEM_ENCRYPT
section and guard it with CONFIG_X86_MEM_ENCRYPT.

Fixes: 439e1757 ("init, x86: Move mem_encrypt_init() into arch_cpu_finalize_init()")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Closes: https://lore.kernel.org/oe-kbuild-all/202306170247.eQtCJPE8-lkp@intel.com/

arch/x86/include/asm/mem_encrypt.h

@@ -17,6 +17,12 @@
 
 #include <asm/bootparam.h>
 
+#ifdef CONFIG_X86_MEM_ENCRYPT
+void __init mem_encrypt_init(void);
+#else
+static inline void mem_encrypt_init(void) { }
+#endif
+
 #ifdef CONFIG_AMD_MEM_ENCRYPT
 
 extern u64 sme_me_mask;
@@ -51,8 +57,6 @@ void __init mem_encrypt_free_decrypted_mem(void);
 
 void __init sev_es_init_vc_handling(void);
 
-void __init mem_encrypt_init(void);
-
 #define __bss_decrypted __section(".bss..decrypted")
 
 #else	/* !CONFIG_AMD_MEM_ENCRYPT */
@@ -85,8 +89,6 @@ early_set_mem_enc_dec_hypercall(unsigned long vaddr, int npages, bool enc) {}
 
 static inline void mem_encrypt_free_decrypted_mem(void) { }
 
-static inline void mem_encrypt_init(void) { }
-
 #define __bss_decrypted
 
 #endif	/* CONFIG_AMD_MEM_ENCRYPT */