Avoid error/warning about implicit function declaration

Avoid error/warning about implicit function declaration

Function xorgGlxCreateVendor() is defined in glx_extinit.h, if this header is
not included, we might get either error or warning. This header also need to
be included after scrninststr.h header as it defines some structures used
in glx_extinit.h

(cherry picked from commit 8c6c584377feba0e3b99eecb3ef33b28cee318cb)