don't skip tools from minimalized build

the switch to <module>_BUILD_PARTS turned += into = as a side effect.
compensate by explicitly including libs & tools.

this went unnoticed so far, because the only repo in which an autotest
would catch it (qttools) didn't even pay attention to the tools
exclusion so far.

Change-Id: Ia97c0b881a7028f6f8f6e855224d5110cf9e2f43
Reviewed-by: Tony Sarajärvi <tony.sarajarvi@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>