Fix OpenGL build on Linux

[elevenfive]

Without this fix, getting errors like this (Ubuntu 25.04 / x86_64)

/usr/bin/ld: CMakeFiles/acidwarp.dir/display.c.o: in function `disp_setPalette':
display.c:(.text+0xe5): undefined reference to `glActiveTexture'
/usr/bin/ld: display.c:(.text+0xf7): undefined reference to `glBindTexture'
/usr/bin/ld: display.c:(.text+0x132): undefined reference to `glTexSubImage2D'
/usr/bin/ld: display.c:(.text+0x140): undefined reference to `glClear'
/usr/bin/ld: display.c:(.text+0x154): undefined reference to `glDrawArrays'