XQuartz: Fix get_proc_address signature

XQuartz: Fix get_proc_address signature

indirect.c:675:28: warning: incompatible pointer types passing 'glx_gpa_proc (*)(const char *)' to parameter of type
      'glx_gpa_proc' (aka 'glx_func_ptr (*)(const char *)') [-Wincompatible-pointer-types,Semantic Issue]
    __glXsetGetProcAddress(&get_proc_address);
                           ^~~~~~~~~~~~~~~~~
../../../glx/glxserver.h:122:42: note: passing argument to parameter 'get_proc_address' here [Semantic Issue]
void __glXsetGetProcAddress(glx_gpa_proc get_proc_address);
                                         ^

Signed-off-by: Jeremy Huddleston Sequoia <jeremyhu@apple.com>