glthread: Fix use of alloca() without #include "c99_alloca.h"

glthread: Fix use of alloca() without #include "c99_alloca.h"

../src/mesa/main/glthread_draw.c: In function ‘_mesa_marshal_MultiDrawElementsBaseVertex’:
../src/mesa/main/glthread_draw.c:812:36: error: implicit declaration of function ‘alloca’; did you mean ‘malloc’? [-Werror=implicit-function-declaration]
  812 |       const GLvoid **out_indices = alloca(sizeof(indices[0]) * draw_count);
      |                                    ^~~~~~
      |                                    malloc
../src/mesa/main/glthread_draw.c:812:36: error: initialization of ‘const GLvoid **’ {aka ‘const void **’} from ‘int’ makes pointer from integer without a cast [-Werror=int-conversion]
cc1: some warnings being treated as errors

Include c99_alloca.h to portably make the alloca() prototype available.

Fixes: 2840bc30
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4920>