Fix preprocessor conditional

This was intended as an if-else-if chain. Make it so.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/library/aesni.h b/library/aesni.h
index 2493998..51b770f 100644
--- a/library/aesni.h
+++ b/library/aesni.h
@@ -67,8 +67,7 @@
  * likely remove the assembly implementation. */
 #if defined(MBEDTLS_HAVE_X86_64)
 #define MBEDTLS_AESNI_HAVE_CODE 1 // via assembly
-#endif
-#if defined(MBEDTLS_AESNI_HAVE_INTRINSICS)
+#elif defined(MBEDTLS_AESNI_HAVE_INTRINSICS)
 #define MBEDTLS_AESNI_HAVE_CODE 2 // via intrinsics
 #endif