gimp: add DEPENDS and PACKAGECONFIG for help-browser

* missing DEPENDS were detected in Martin's world with test-dependencies
  (libxmu & libxpm will still apear in distroless environments)
* help-browser depends on webkit-gtk which takes very long to build so disable
  it by default

Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>