Makefile: Disable parallel make

Disable parallel make, because the Makefile is written funny. Now it
won't fail if make is run with -j # (e.g. because MAKEFLAGS includes
-j #).

Makefile

+.NOTPARALLEL:
+
 gitlab_repo = https://gitlab.com/gitlab-org/gitlab-ce.git
 gitlab_shell_repo = https://gitlab.com/gitlab-org/gitlab-shell.git
 gitlab_shell_clone_dir = go-gitlab-shell/src/gitlab.com/gitlab-org/gitlab-shell