games/stratagus: fix build on GCC architectures

-Wno-c++11-narrowing is only for clang.

gcc needs -Wno-error:
In file included from /wrkdirs/usr/ports/games/stratagus/work/stratagus-2.4.2/src/include/video.h:46,

from /wrkdirs/usr/ports/games/stratagus/work/stratagus-2.4.2/src/action/action_attack.cpp:60:

/usr/local/include/SDL/SDL_opengl.h:116: error: "GL_GLEXT_VERSION" redefined [-Werror]

116 | #define GL_GLEXT_VERSION 29
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