git:ignore: Add tex_build

When compiling a document from inside vim, build files will be written
to tex_build. Ignore them. See .config/vim/ftplugin/tex.vim

.config/git/ignore

@@ -23,3 +23,6 @@ __pycache__/
 
 # Ignore python virtual environment
 /venv/
+
+# Ignore latex build folder
+tex_build/