powerpc/pkeys: Fix build error with PPC_MEM_KEYS disabled

IS_ENABLED() instead of #ifdef still requires variable declaration.
In this specific case, default_uamor is declared in asm/pkeys.h which
is only included if PPC_MEM_KEYS is enabled.

arch/powerpc/mm/book3s64/hash_utils.c: In function ‘hash__early_init_mmu_secondary’:
arch/powerpc/mm/book3s64/hash_utils.c:1119:21: error: ‘default_uamor’ undeclared (first use in this function)
 1119 |   mtspr(SPRN_UAMOR, default_uamor);
      |                     ^~~~~~~~~~~~~

Fixes: 6553fb79 ("powerpc/pkeys: Fix boot failures with Nemo board (A-EON AmigaOne X1000)")
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200817103301.158836-1-aneesh.kumar@linux.ibm.com

arch/powerpc/mm/book3s64/hash_utils.c

@@ -1115,8 +1115,10 @@ void hash__early_init_mmu_secondary(void)
 			&& cpu_has_feature(CPU_FTR_HVMODE))
 		tlbiel_all();
 
-	if (IS_ENABLED(CONFIG_PPC_MEM_KEYS) && mmu_has_feature(MMU_FTR_PKEY))
+#ifdef CONFIG_PPC_MEM_KEYS
+	if (mmu_has_feature(MMU_FTR_PKEY))
 		mtspr(SPRN_UAMOR, default_uamor);
+#endif
 }
 #endif /* CONFIG_SMP */