mesa/util: add missing va_end() after va_copy()

MSDN:
"va_end must be called on each argument list that's initialized
 with va_start or va_copy before the function returns."

Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107810


Fixes: c6267ebd "gallium/util: Stop bundling our snprintf implementation."
Signed-off-by: Andrii Simiklit <andrii.simiklit@globallogic.com>
(cherry picked from commit 2930b76c)