Update .gitignore

.gitignore

-*.aux
-*.bbl
-*.blg
-*.fdb_latexmk
-*.fls
-*.log
-*.nav
-*.out
-*.pdf.deps
-*.snm
-*.synctex.gz
-*.toc
-.deps/
\ No newline at end of file
+*/**/build
+build/
+!build/*.pdf
\ No newline at end of file