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

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.
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>

Gbp-Pq: Name cross_build_mysql.diff

src/plugins/sqldrivers/configure.json

@@ -67,6 +67,7 @@
             },
             "headers": "mysql.h",
             "sources": [
+                { "type": "pkgConfig", "args": "mysqlclient" },
                 { "type": "mysqlConfig", "query": "--libs_r", "cleanlibs": true },
                 { "type": "mysqlConfig", "query": "--libs", "cleanlibs": true },
                 { "type": "mysqlConfig", "query": "--libs_r", "cleanlibs": false },