package/pkg-qmake: add <pkg>_SYNC_QT_HEADERS support

Some qmake based packages need to call the syncqt.pl script before
building to have a properly populated "include" directory inside the
package build tree.

This script is normally automatically executed by qmake when a source
tree is from a git clone: buildroot purges the .git directory hence
the script never runs, and we need to call it explicitly. Setting
<pkg>_SYNC_QT_HEADERS = YES will force calling this script.

Signed-off-by: Angelo Compagnucci <angelo@amarulasolutions.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>