Do not build av1_rc_qmode as OBJECT lib

This is to be compatible with cmake older than 3.12

Bug: aomedia:3301
Change-Id: Ie1720d65eff75b1fa0cfd4d55e41e3ea2b07e829
diff --git a/CMakeLists.txt b/CMakeLists.txt
index a0fa444..44c83e6 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -323,7 +323,7 @@
            "${AOM_ROOT}/av1/ratectrl_qmode.h"
            "${AOM_ROOT}/av1/ratectrl_qmode.cc" "${AOM_ROOT}/av1/ducky_encode.h"
            "${AOM_ROOT}/av1/ducky_encode.cc")
-  add_library(av1_rc_qmode OBJECT ${AOM_AV1_RC_QMODE_SOURCES})
+  add_library(av1_rc_qmode ${AOM_AV1_RC_QMODE_SOURCES})
   target_link_libraries(av1_rc_qmode ${AOM_LIB_LINK_TYPE} aom)
   if(NOT MSVC AND NOT APPLE)
     target_link_libraries(av1_rc_qmode ${AOM_LIB_LINK_TYPE} m)