Define missing `NANOVG_GLEW` for Linux configurations

[merkys]

When building nanovg on Debian unstable with -Werror=implicit-function-declaration in CFLAGS, three of the examples fail to build due to undefined functions. It seems that NANOVG_GLEW definition is missing in Linux configurations of these three examples despite being present in Windows configurations. This PR fixes the issue by adding NANOVG_GLEW to Linux configurations for these three examples.