Make libpanes a static library

Summary:
add_library uses the value of BUILD_SHARED_LIBS to decide wether to
build a static or dynamic lib. If it is on, libpane is built but not
installed, which causes kookbook to fail on start, complaining about
"error while loading shared libraries: libpanes.so: cannot open
shared object file: No such file or directory".
Instead make it a static lib.

Test Plan: kookbook starts fine.

Differential Revision: https://phabricator.kde.org/D17633