mesa: Resolve GCC sign-compare warning.

mesa: Resolve GCC sign-compare warning.

mesa/src/mesa/program/prog_to_nir.c: In function 'setup_registers_and_variables':
/mesa/src/mesa/program/prog_to_nir.c:1059:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
    for (int i = 0; i < c->prog->NumTemporaries; i++) {
                      ^

Signed-off-by: Rhys Kidd <rhyskidd@gmail.com>
Reviewed-by: Thomas Helland <thomashelland90@gmail.com>
Reviewed-by: Jan Vesely <jan.vesely@rutgers.edu>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>