av1_inv_txfm_neon.c: add missing prototype

For av1_lowbd_inv_txfm2d_add_neon(). This function is exposed to allow
testing in av1_inv_txfm2d_test.cc.

Bug: aomedia:3416
Change-Id: Ied05d3b6589c9c38ef14f204cea35294fb229cd7

av1/common/arm/av1_inv_txfm_neon.c

diff --git a/av1/common/arm/av1_inv_txfm_neon.c b/av1/common/arm/av1_inv_txfm_neon.c
index f15d473..06168cd 100644
--- a/av1/common/arm/av1_inv_txfm_neon.c
+++ b/av1/common/arm/av1_inv_txfm_neon.c
@@ -4177,6 +4177,11 @@
   }
 }
 
+// This function is used by av1_inv_txfm2d_test.cc.
+void av1_lowbd_inv_txfm2d_add_neon(const int32_t *input, uint8_t *output,
+                                   int stride, TX_TYPE tx_type, TX_SIZE tx_size,
+                                   int eob);
+
 void av1_lowbd_inv_txfm2d_add_neon(const int32_t *input, uint8_t *output,
                                    int stride, TX_TYPE tx_type, TX_SIZE tx_size,
                                    int eob) {