Fix a Wmissing-declarations warning when compiling with MinGW

Namely, this warning: http://build.opencv.org/builders/win_x64%5Brelease%5D--%5Bsse_%3D_ON%5D--%5Btbb_%3D_OFF%5D--%5Bgpu_%3D_OFF%5D--%5Bcompiler_%3D_mingw%5D--%5Btests_%3D_ON%5D-/builds/1199/steps/compile%20release%2064%20mingw%20shared%20n/logs/build%20summary%20%28e%3A%200%2C%20w%3A%201%29