Fix use of alloca() without #include <c99_alloca.h>

Fix use of alloca() without #include <c99_alloca.h>

Fix use of alloca() without #include <c99_alloca.h> in 1da345e5

vbo/vbo_context.c: In function '_vbo_draw_indirect':
vbo/vbo_context.c:284:34: error: implicit declaration of function 'alloca' [-Werror=implicit-function-declaration]
       struct _mesa_prim *space = alloca(draw_count*sizeof(struct _mesa_prim));
                                  ^~~~~~
vbo/vbo_context.c:284:34: warning: initialization makes pointer from integer without a cast [-Wint-conversion]

Signed-off-by: Jon Turney <jon.turney@dronecode.org.uk>
Reviewed-by: Mathias Fröhlich <mathias.froehlich@web.de>