vala: cleanup DEPENDS

vala: cleanup DEPENDS

Patch cleans up and fixes dependencies of 'vala' in the following way:

* remove deps on 'gtk+' + 'dbus'; these packages are not required and
  'gkt+' is high barrier preventing using 'vala' on headless systems

* 'bison-native' is required both on native and target builds; 'DEPENDS'
  and 'DEPENDS_virtclass-native' and 'DEPENDS' can be merged hence

* 'flex-native' is required

* although 'xsltproc' (from libxslt-native) is optional, a dependency
  on it was added to make builds deterministic.  An alternative method
  might be 'EXTRA_OECONF += "ac_cv_path_XSLTPROC=:"' but this is error
  prone as it depends on internals of the build system.

(From OE-Core rev: 8834e6b2aa70a2b3d43550d3912e16b8f5c12d7e)

Signed-off-by: Enrico Scholz <enrico.scholz@sigma-chemnitz.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>