Fix build

In make, appending to a non-existant variable using += results in
delayed evalution of the RHS.  If another Android.mk file assigns to GEN
between this one and when ALL_COPIED_HEADERS is evaluated, it will see
the new value of GEN.

Replace += with :=.

Change-Id: I77400bf7eed788cae89dcd9fd690717db4780f39
1 file changed