Update Gitignore to ignore generated files.

.gitignore

@@ -18,3 +18,18 @@ test*.json
 results
 /compile_commands.json
 /.cache
+build/.cmake/
+build/CMakeCache.txt
+build/CMakeFiles/
+build/Makefile
+build/SPIRV-Tools*
+build/cmake_install.cmake
+build/compile_commands.json
+build/filament/
+build/include/
+build/libs/
+build/mac/ninja
+build/samples/
+build/shaders/
+build/third_party/
+build/tools/