emulators/yuzu: unbreak after 85a65c9557ca

• SDL2 2.0.14 or newer not found, falling back to externals.

CMake Error at externals/CMakeLists.txt:54 (add_subdirectory):

The source directory

  /wrkdirs/usr/ports/emulators/yuzu/work/yuzu-2e268abe4/externals/SDL

does not contain a CMakeLists.txt file.

Reported by: pkg-fallout

(cherry picked from commit d02aa5e9ce6b357bdb5b8cd494d0f6a4a1249d7e)