- fix building with system gtest

Index: cmake/utils.cmake
--- cmake/utils.cmake.orig
+++ cmake/utils.cmake
@@ -18,7 +18,7 @@ function (shaderc_use_gmock TARGET)
   target_include_directories(${TARGET} PRIVATE
     ${gmock_SOURCE_DIR}/include
     ${gtest_SOURCE_DIR}/include)
-  target_link_libraries(${TARGET} PRIVATE gmock gtest_main)
+  target_link_libraries(${TARGET} PRIVATE gmock gtest gtest_main)
 endfunction(shaderc_use_gmock)
 
 function(shaderc_default_c_compile_options TARGET)
