Update .gitignore

add `build` directory since usually CMake users use
`cmake -S. -Bbuild -DENABLE_TESTING=ON`
diff --git a/.gitignore b/.gitignore
index 49400b7..6285424 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,3 +1,4 @@
 cmake_build/
+build/
 
 *.swp