REDUCED_ENCODER_BORDER: Fix compiler warning

Earlier compiler was warning about missing definition for 'warn()'.

BUG=aomedia:2108

Change-Id: I2742d7edae1485363df3f2d0a5bfd5dcb552e408
diff --git a/av1/av1_cx_iface.c b/av1/av1_cx_iface.c
index 3cb1567..43a6028 100644
--- a/av1/av1_cx_iface.c
+++ b/av1/av1_cx_iface.c
@@ -26,6 +26,10 @@
 #include "av1/encoder/encoder.h"
 #include "av1/encoder/firstpass.h"
 
+#if CONFIG_REDUCED_ENCODER_BORDER
+#include "common/tools_common.h"
+#endif  // CONFIG_REDUCED_ENCODER_BORDER
+
 #define MAG_SIZE (4)
 #define MAX_NUM_ENHANCEMENT_LAYERS 3