Makefile: Express rebar3 profile using "as profile"

This allows to use REBAR_PROFILE to add another profile.
Right now it doesn't seem specially useful, but it's possible:
  REBAR_PROFILE=dev make prod
  REBAR_PROFILE=translations make dev

https://rebar3.org/docs/configuration/profiles/

Makefile.in

@@ -160,7 +160,7 @@ endif
   XREFOPTIONS=
   CLEANARG=--all
   REBARREL=$(REBAR) as prod tar
-  REBARDEV=REBAR_PROFILE=dev $(REBAR) release
+  REBARDEV=$(REBAR) as dev release
   RELIVECMD=$(REBAR) relive
   REL_LIB_DIR = _build/dev/rel/ejabberd/lib
   COPY_REL_TARGET = dev