graphics/qgis: Readd patch to fix flex version

For FreeBSD versions < 1300501 the existing version of FLEX in base is
unsufficient. Readd old patch to install the ports version of FLEX.

CMake Error at /usr/local/share/cmake/Modules/FindPackageHandleStandardArgs.cmake:230

(message): Could NOT find FLEX: Found unsuitable version "2.5.37",
but required is at least "2.6" (found /usr/bin/flex)

Call Stack (most recent call first):

/usr/local/share/cmake/Modules/FindPackageHandleStandardArgs.cmake:598
  (_FPHSA_FAILURE_MESSAGE)
/usr/local/share/cmake/Modules/FindFLEX.cmake:269
  (FIND_PACKAGE_HANDLE_STANDARD_ARGS)
CMakeLists.txt:339 (find_package)

Thanks to Martin Birgmeier <d8zNeCFG@aon.at> for reporting.

PR: 274793