android_make: export_includes need to depend on generated output
android_make: export_includes need to depend on generated output

The export_includes file produced for generated libraries needs to
depend on the files being created by the command(s). This forces make
(and ninja) to build the generated library before attempting to build
something that needs to use generated headers.

Change-Id: I57e5d30abe49ae1a75728c8527a0baed85ccf9db
Signed-off-by: David Kilroy <159cd46f289d7961b38b0171b53c05be4aaef0b6@arm.com>

ARM-software/bob-build,ARM-software/bob-build,ARM-software/bob-build,ARM-software/bob-build,ARM-software/bob-build