AESNI: improve comments on some guards in aesni.h

Signed-off-by: Pengyu Lv <pengyu.lv@arm.com>
diff --git a/library/aesni.h b/library/aesni.h
index ba14290..5c5a026 100644
--- a/library/aesni.h
+++ b/library/aesni.h
@@ -165,6 +165,6 @@
 #endif
 
 #endif /* MBEDTLS_AESNI_HAVE_CODE */
-#endif  /* MBEDTLS_AESNI_C */
+#endif  /* MBEDTLS_AESNI_C && (MBEDTLS_ARCH_IS_X64 || MBEDTLS_ARCH_IS_X86) */
 
 #endif /* MBEDTLS_AESNI_H */