scripts/dtc: Add missing fdtoverlay to gitignore

Commit 0da6bcd9 ("scripts: dtc: Build fdtoverlay tool") enabled
building fdtoverlay, but failed to add it to .gitignore.

Also add a note to keep hostprogs in sync with .gitignore.

Fixes: 0da6bcd9 ("scripts: dtc: Build fdtoverlay tool")
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>

scripts/dtc/.gitignore

 # SPDX-License-Identifier: GPL-2.0-only
 dtc
+fdtoverlay

scripts/dtc/Makefile

 # SPDX-License-Identifier: GPL-2.0
 # scripts/dtc makefile
 
+# *** Also keep .gitignore in sync when changing ***
 hostprogs-always-$(CONFIG_DTC)		+= dtc fdtoverlay
 hostprogs-always-$(CHECK_DT_BINDING)	+= dtc