framework: add hook to check indentation of conditionals of Mk/ files

This adds a further pre-commit hook to enforce indentations in Mk/*

Usage:
> vim Mk/Uses/tcl.mk
[add/remove some whitespaces between a . and an if]
> git add Mk/Uses/tcl.mk
> git commit -m "Mk/Uses/tcl.mk: test hook"
[pre-commit] tcl.mk is not properly indented -- please use /tmp/check_indentations-tcl.mk.LShfR7TD48/tcl.mk which was created using Tools/scripts/indent_make_if.pl

Reminder: to make use of the hooks provided by the ports tree use the
following from [1]

Add the prepare-commit-msg hook to the repository.

To make use of it, the easiest way is to run:

  git config --add core.hooksPath .hooks

[1] bbc2474ef7a65eb8561c8ecf7af80c2bfed1f248

Reviewed by: portmgr (rene)
Differential Revision: https://reviews.freebsd.org/D35041