[cmake] fix libdvd depends build

[lrusak]

Description

libdvd{read,css,nav} should be built when KODI_DEPENDSBUILD is set, not the other way around.

Instead of the NOT I can flop the logic around instead if desired.

Motivation and Context

libdvd{read,css,nav} should not be downloaded/built when not doing a depends build

How Has This Been Tested?

build tested in LibreELEC

With this it now finds and links the correct libraries

DVDREAD_LIBRARY:FILEPATH=/home/lukas/libreelec/build.LibreELEC-RPi2.arm-8.0-devel/toolchain/armv7ve-libreelec-linux-gnueabi/sysroot/usr/lib/libdvdread.a
DVDCSS_LIBRARY:FILEPATH=/home/lukas/libreelec/build.LibreELEC-RPi2.arm-8.0-devel/toolchain/armv7ve-libreelec-linux-gnueabi/sysroot/usr/lib/libdvdcss.a
DVDNAV_LIBRARY:FILEPATH=/home/lukas/libreelec/build.LibreELEC-RPi2.arm-8.0-devel/toolchain/armv7ve-libreelec-linux-gnueabi/sysroot/usr/lib/libdvdnav.a

Screenshots (if appropriate):

Types of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the Code guidelines of this project
• My change requires a change to the documentation, either Doxygen or wiki
• I have updated the documentation accordingly
• I have read the CONTRIBUTING document
• I have added tests to cover my change
• All new and existing tests passed

[hudokkow]

travis doesn't like it.

[lrusak]

Do we not include libdvd* on the system for Linux builds?

[wsnipex]

This is wrong!

[lrusak]

So if it's a depends build it doesn't build libdvd*???

And if it's not a depends build it builds libdvd*??

How does that make sense?

[wsnipex]

KODI_DEPENDSBUILD is true when unified depends are used. that means all depends have been built before kodi