Makefile: correct make 4.3+ workaround for 2019.02.x

Commit bdb4a9e110b67ebe3336a (Makefile: work around a bug in newly released
make 4.3) backported a fix from master, but it doesn't work on 2019.02.x as
the br2-external file is called .br-external.mk, not .br2-external.mk.

Adjust the workaround to match.

Reported-by: Mircea GLIGA <mgliga@bitdefender.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

Makefile

@@ -190,7 +190,7 @@ $(error $(BR2_EXTERNAL_ERROR))
 endif
 
 # Workaround bug in make-4.3: https://savannah.gnu.org/bugs/?57676
-$(BASE_DIR)/.br2-external.mk:;
+$(BASE_DIR)/.br-external.mk:;
 
 # To make sure that the environment variable overrides the .config option,
 # set this before including .config.