Fix wrong string GLFW3_FOUND instead of correct glfw3_FOUND

[traversaro]

Follow up of #52 .

[traversaro]

Without this fix, nothing is actually build, see the log from CI:

[1/8] Building CXX object _deps/glfont-build/CMakeFiles/GLFont.dir/src/FontAtlas.cpp.o
[2/8] Building CXX object _deps/glfont-build/CMakeFiles/GLFont.dir/src/GLFont.cpp.o
[3/8] Building CXX object _deps/glfont-build/CMakeFiles/GLFont.dir/src/GLUtils.cpp.o
[4/8] Building CXX object _deps/glfont-build/CMakeFiles/GLFont.dir/src/FTLabel.cpp.o
[5/8] Linking CXX shared library lib/libGLFont.so.0.0.1
[6/8] Creating library symlink lib/libGLFont.so
[7/8] Building CXX object src/vendor/stb/CMakeFiles/stb_image.dir/stb_image.cpp.o
[8/8] Linking CXX static library lib/libstb_image.a

while after this fix the device is installed.