meson: get opengl compilation flags from OPENGL_CFLAGS

The opengl compilation flags were added to QEMU_CFLAGS.  We do not
want them to be added to all compilation commands, so export them
also via OPENGL_CFLAGS rather than via QEMU_CFLAGS.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>