use add_plugin in dump

for elektra-multiconfig-cmake test
should fix

/usr/bin/ld: cannot find -lelektra
collect2: error: ld returned 1 exit status
src/plugins/dump/CMakeFiles/elektra-dump.dir/build.make:90: recipe for
target 'src/plugins/dump/libelektra-dump.so' failed
make[2]: *** [src/plugins/dump/libelektra-dump.so] Error 1
CMakeFiles/Makefile2:1126: recipe for target
'src/plugins/dump/CMakeFiles/elektra-dump.dir/all' failed