Install libcouchbase/assert.h header

Change-Id: I2b8f26fb6d9824bea4a2d37022506750c912f183
Reviewed-on: http://review.couchbase.org/28205
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>