Fix CONFIG_BGSPRITE builds with CMake.

Add av1/encoder/bgsprite.[ch] to the CMake build.

BUG=aomedia:683

Change-Id: Ic90c81c724be6814a3534f2f612173ac325d4423

av1/av1.cmake

diff --git a/av1/av1.cmake b/av1/av1.cmake
index 084d5e4..4ec1ae2 100644
--- a/av1/av1.cmake
+++ b/av1/av1.cmake
@@ -305,6 +305,13 @@
       "${AOM_ROOT}/av1/decoder/accounting.h")
 endif ()
 
+if (CONFIG_BGSPRITE)
+  set(AOM_AV1_ENCODER_SOURCES
+      ${AOM_AV1_ENCODER_SOURCES}
+      "${AOM_ROOT}/av1/encoder/bgsprite.c"
+      "${AOM_ROOT}/av1/encoder/bgsprite.h")
+endif ()
+
 if (CONFIG_GLOBAL_MOTION)
   set(AOM_AV1_ENCODER_SOURCES
       ${AOM_AV1_ENCODER_SOURCES}