Fix warning when building without Zstd support
This MR fixes a build warning when building KArchive without Zstd support
/Users/claucambra/Downloads/karchive/src/kcompressiondevice.cpp:162:13: warning: enumeration value 'Zstd' not handled in switch [-Wswitch]
switch (type) {
^~~~
/Users/claucambra/Downloads/karchive/src/kcompressiondevice.cpp:162:13: note: add missing switch cases
switch (type) {
^
1 warning generated.