Since 'XComponentContext' and 'DECL_STATIC_LINK' are used,
add the corresponding includes.

Otherwise a successful build depends on the header being
included after other headers that make sure that compilation
still works.

An error could be provoked e.g. by moving the
'#include <gtk/fpicker/SalGtkFilePicker.hxx>' in
'vcl/unx/gtk/fpicker/SalGtkFilePicker.cxx' above all
other existing includes.

Change-Id: Ib23f8b714fb971238eab28a8d4d02414a0b13812
Reviewed-on: https://gerrit.libreoffice.org/65208
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>