package/qt5virtualkeyboard: fix install for Qt 5.12.x

package/qt5virtualkeyboard: fix install for Qt 5.12.x

With Qt 5.12.x only handwriting/lipi-toolkit needs 3rdparty parts
installation (with Qt 5.6.x although zn_CZ/pinyin and zh_TW tcime).

Fixes:
  - https://bugs.busybox.net/show_bug.cgi?id=12456

  cp: cannot stat '.../host/arm-buildroot-linux-gnueabihf/sysroot/usr/qtvirtualkeyboard': No such file or directory

Also fix the way we test the variable: we very seldomly use ifdef,
instead we usually test for equality.

Reported-by: Sam Petrocelli <sam.petrocelli@gmail.com>
Signed-off-by: Peter Seiderer <ps.report@gmx.net>
[yann.morin.1998@free.fr: also fix the way we test the variable]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>