MT#61993 build: Declare all to be the default goal

This makes sure the target is the default, regardless of the place where
it gets declared, so that we do not get surprises due to targets
declared in includes that might happened to be performed before the
default target.

Change-Id: I2fab47ccb46d68dc56332acef966e369c5183c07
(cherry picked from commit 5cb1002949)

Makefile

@@ -1,3 +1,5 @@
+.DEFAULT_GOAL := all
+
 RTPENGINE_ROOT_DIR=.
 with_transcoding ?= yes
 

lib/common.Makefile

@@ -1,3 +1,5 @@
+.DEFAULT_GOAL := all
+
 include ../lib/lib.Makefile
 
 CFLAGS += -fPIE