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

[12/60] Compiling C object 'src/gallium/auxiliary/eb820e8@@gallium@sta/rbug_rbug_texture.c.o'.
FAILED: src/gallium/auxiliary/eb820e8@@gallium@sta/rbug_rbug_texture.c.o
[...]
../src/gallium/auxiliary/rbug/rbug_texture.c: In function 'rbug_send_texture_info_reply':
../src/gallium/auxiliary/rbug/rbug_texture.c:302:21: error: implicit declaration of function 'alloca'; did you mean 'malloc'? [-Werror=implicit-function-declaration]
  uint32_t *height = alloca(sizeof(uint32_t) * height_len);
                     ^~~~~~
                     malloc
../src/gallium/auxiliary/rbug/rbug_texture.c:302:21: warning: initialization makes pointer from integer without a cast [-Wint-conversion]
../src/gallium/auxiliary/rbug/rbug_texture.c:303:20: warning: initialization makes pointer from integer without a cast [-Wint-conversion]
  uint32_t *depth = alloca(sizeof(uint32_t) * height_len);
                    ^~~~~~
cc1: some warnings being treated as errors

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

Fixes: 6174cba7 ("rbug: fix transmitted texture sizes")

See also: 498d9d0f, adfb9c5c, fc8139b1