Fix encoder use of 16-bit operating path

When compiled with --disable-lowbitdepth, the decoder uses the generic
16-bit operating path, regardless of the input video profile.
The encoder should also use the generic 16-bit operating path if the
low-bitdepth operating path has been disabled at build time.

Change-Id: I421cc774b45d2c76bea8730cc8726224cb4aba1e
diff --git a/aomenc.c b/aomenc.c
index c26d346..62aae01 100644
--- a/aomenc.c
+++ b/aomenc.c
@@ -1273,11 +1273,15 @@
     }
   }
 #if CONFIG_HIGHBITDEPTH
+#if CONFIG_LOWBITDEPTH
   if (strcmp(global->codec->name, "av1") == 0 ||
       strcmp(global->codec->name, "av1") == 0) {
     config->use_16bit_internal =
         test_16bit_internal | (config->cfg.g_profile > 1);
   }
+#else
+  config->use_16bit_internal = 1;
+#endif
 #endif
   return eos_mark_found;
 }