Add support for kFreeBSD to GRASS plugin.

[sebastic]

As reported in Debian Bug #824079, builds of qgis on kFreeBSD failed:

  /«BUILDDIR»/qgis-2.14.2+dfsg/src/plugins/grass/qtermwidget/kpty.cpp: In member function 'bool KPty::tcGetAttr(termios*) const':
  /«BUILDDIR»/qgis-2.14.2+dfsg/src/plugins/grass/qtermwidget/kpty.cpp:126:44: error: 'TCGETS' was not declared in this scope
   #  define _tcgetattr(fd, ttmode) ioctl(fd, TCGETS, (char *)ttmode)
                                              ^
  /«BUILDDIR»/qgis-2.14.2+dfsg/src/plugins/grass/qtermwidget/kpty.cpp:647:12: note: in expansion of macro '_tcgetattr'
       return _tcgetattr(d->masterFd, ttmode) == 0;
              ^
  /«BUILDDIR»/qgis-2.14.2+dfsg/src/plugins/grass/qtermwidget/kpty.cpp: In member function 'bool KPty::tcSetAttr(termios*)':
  /«BUILDDIR»/qgis-2.14.2+dfsg/src/plugins/grass/qtermwidget/kpty.cpp:136:44: error: 'TCSETS' was not declared in this scope
   #  define _tcsetattr(fd, ttmode) ioctl(fd, TCSETS, (char *)ttmode)
                                              ^
  /«BUILDDIR»/qgis-2.14.2+dfsg/src/plugins/grass/qtermwidget/kpty.cpp:654:12: note: in expansion of macro '_tcsetattr'
       return _tcsetattr(d->masterFd, ttmode) == 0;
              ^

This is caused by the lack of the GNU/kFreeBSD define in the list of BSD variants to test.

Adding __FreeBSD_kernel__ as documented in the kFreeBSD FAQ in src/plugins/grass/qtermwidget/kpty.cpp resolves the issue.

[sebastic]

The Travis failures are unexpected, and don't seem to be caused by the changes in this PR.