Remove export header from static install

Summary: When building the "install" target with a static Kirigami build an error occurs due to the missing file kirigami2_export.h.

Test Plan:
Download Kirigami sources, create build directory, create install directory, and then something similar to:

INSTALL_DIR=$HOME/tmp-install
QT_INSTALL_DIR=$HOME/qt-5.12.6
ECM_DIR=$HOME/tmp-install  # Assume ECM was also "make installed" into this dir
KIRIGAMI_SRC=$HOME/kirigami

cmake -DBUILD_SHARED_LIBS=OFF -DCMAKE_INSTALL_PREFIX=$INSTALL_DIR -DCMAKE_PREFIX_PATH=$QT_INSTALL_DIR;$ECM_DIR $KIRIGAMI_SRC
cmake --build . --target install

Before the patch this will fail because of missing kirigami2_export.h file.

Reviewers: Kirigami, mart, apol

Reviewed By: Kirigami, mart

Subscribers: jbbgameich, plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D26956