SDL_x11sym.h: add symbols for _XGetRequest

While intoducing to yocto few libraries complain:

| /home/tmp/oe-core-eglibc/sysroots/overo/usr/lib/libSDL.so: undefined reference to `_XGetRequest'

This is a backport from libsdl-1.2.15.

Signed-off-by: Andreas M?ller <schnitzeltony@googlemail.com>