glx: Suppress unused variable warning for cmdlen

glx: Suppress unused variable warning for cmdlen

No functional change.  In the function
__indirect_glAreTexturesResident(), the variable cmdlen is only used
if USE_XCB is not defined.  This patch avoids a compile warning in the
event that USE_XCB is defined.

v2: just move cmdlen declaration inside the #else part.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>