package/kodi: Fix compilation of kodi-pvr-vbox with gcc6

package/kodi: Fix compilation of kodi-pvr-vbox with gcc6

Fixes a compile error not yet caught by autobuilders:

In file included from /home/bernd/buildroot/br2/output/build/kodi-pvr-vbox-15e864d160da5a051e18aef06f3a53e49808be02/src/client.cpp:24:0:
/home/bernd/buildroot/br2/output/host/usr/i586-buildroot-linux-uclibc/sysroot/usr/include/kodi/xbmc_pvr_dll.h:423:63: error: 'SEEK_SET' was not declared in this scope
   long long SeekLiveStream(long long iPosition, int iWhence = SEEK_SET);
                                                               ^~~~~~~~
/home/bernd/buildroot/br2/output/host/usr/i586-buildroot-linux-uclibc/sysroot/usr/include/kodi/xbmc_pvr_dll.h:510:67: error: 'SEEK_SET' was not declared in this scope
   long long SeekRecordedStream(long long iPosition, int iWhence = SEEK_SET);
                                                                   ^~~~~~~~
CMakeFiles/pvr.vbox.dir/build.make:62: recipe for target 'CMakeFiles/pvr.vbox.dir/src/client.cpp.o' failed

using this defconfig

BR2_TOOLCHAIN_BUILDROOT_WCHAR=y
BR2_BINUTILS_VERSION_2_26_X=y
BR2_GCC_VERSION_6_X=y
BR2_TOOLCHAIN_BUILDROOT_CXX=y
BR2_INIT_NONE=y
BR2_PACKAGE_KODI=y
BR2_PACKAGE_KODI_PVR_VBOX=y
BR2_PACKAGE_MESA3D=y
BR2_PACKAGE_MESA3D_DRI_DRIVER_I965=y
BR2_PACKAGE_MESA3D_OPENGL_EGL=y
BR2_PACKAGE_XORG7=y

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>