cmake: fix -DCONFIG_GCOV=1

the coverage flags are required at link time as well as compile time

Change-Id: Ic581f8ce747615f1fa1dc13d989d6e233e982572

build/cmake/aom_configure.cmake

diff --git a/build/cmake/aom_configure.cmake b/build/cmake/aom_configure.cmake
index 7a0d239..518c2d3 100644
--- a/build/cmake/aom_configure.cmake
+++ b/build/cmake/aom_configure.cmake
@@ -191,6 +191,7 @@
 
 if (CONFIG_GCOV)
   message("--- Testing for CONFIG_GCOV support.")
+  require_linker_flag("-fprofile-arcs -ftest-coverage")
   require_compiler_flag("-fprofile-arcs -ftest-coverage" YES)
 endif ()