.gitignore: Ignore build directory

That directory is created by `make all` from ocrd_all and should be ignored
to get a clean `git status`.

Signed-off-by: Stefan Weil <sw@weilnetz.de>

.gitignore

@@ -1,3 +1,4 @@
+/build
 *.egg-info
 __pycache__
 tests/**/*.jpg