extra/tools: ninja: force python3

It seems that python3 is already specified in all the VLC
contribs/tools.

Fixes build on systems not having a shortcut python -> python3 (macOS
for example).

(cherry picked from commit 4e90abceb4)
Signed-off-by: Thomas Guillem <thomas@gllm.fr>

extras/tools/tools.mak

@@ -414,7 +414,7 @@ ninja: ninja-$(NINJA_VERSION).tar.gz
 	$(MOVE)
 
 .buildninja: ninja
-	(cd $<; ./configure.py --bootstrap && mv ninja $(PREFIX)/bin/)
+	(cd $<; python3 ./configure.py --bootstrap && mv ninja $(PREFIX)/bin/)
 	touch $@
 
 CLEAN_PKG += ninja