Minor fix which helps Xcode generated project build.

Issue is that the add_custom_command references gencode/genconfig via
a path, yet on Xcode build all output goes into
build/ep-engine/Debug|Release/ thus gencode/genconfig fail dependency
checks.

Using just the add_excutable name of gencode/genconfig allows cmake
to use the correct path.

Change-Id: Ie55124487bffe80bc2847b67977738e67d8da855
Reviewed-on: http://review.couchbase.org/40825
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>