I think error can removed by using

CONFIG += release

QTPLUGIN: QSQLMYSQLS

INCLUDEPATH += C:\Program Files (x86)\MySQL\mysql-connector-c-6.1.11-win32\include

win32: LIBS += C:\Program Files (x86)\MySQL\mysql-connector-c-6.1.11-win32\include\lib\ -llibmysql

in your .pro file

if not working then try by adding

win32:CONFIG(release, debug|release): LIBS += C:\Program Files (x86)\MySQL\mysql-connector-c-6.1.11-win32\include\lib\ -llibmysql
else:win32:CONFIG(debug, debug|release): LIBS += C:\Program Files (x86)\MySQL\mysql-connector-c-6.1.11-win32\include\lib\ -llibmysqld