call pkgconfig in order to be able to cross build qtbase with MySql.

call pkgconfig in order to be able to cross build qtbase with MySql.

Bug-Debian: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=971604
Forwarded: not-needed
Reviewed-by: Lisandro Damián Nicanor Pérez Meyer <lisandro@debian.org>
Qt's build system calls mysql_config... which won't work in a cross build
environment like Debian's, as it will throw an exec format error.

In order to solve this call pkgconfig and use mysqlclient.pc.

Gbp-Pq: Name cross_build_mysql.diff