gitignore: ignore utils/brmake log output named `br.log`

The `utils/brmake` tool runs `make all` with logs put into `br.log`.
That file is therefore the result of a build and committing it never
makes sense, neither upstream nor on any other remote/branch.

    ⟩ git status --short
    ⟩ make beaglebone_defconfig
    ⟩ ./utils/brmake
    ⟩ git status --short
    ?? br.log

Add a new `/br.log` entry in the root `.gitignore` file.
Append to the end because no ordering logic was found.

Signed-off-by: Théo Lebrun <theo.lebrun@bootlin.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

.gitignore

@@ -13,3 +13,4 @@
 *.rej
 *~
 *.pyc
+/br.log