ac/debug: use util_strchrnul() to fix android build error

ac/debug: use util_strchrnul() to fix android build error

Similar to e09d04cd56 "radeonsi: use util_strchrnul() to fix android build error"

Android Bionic does not support strchrnul() string function,
gallium auxiliary util/u_string.h provides util_strchrnul()

This change avoids the following warning and error:

external/mesa/src/amd/common/ac_debug.c:501:15: warning: implicit declaration of function 'strchrnul' is invalid in C99
                char *end = strchrnul(out, '\n');
                            ^
external/mesa/src/amd/common/ac_debug.c:501:9: error: incompatible integer to pointer conversion initializing 'char *' with an expression of type 'int'
                char *end = strchrnul(out, '\n');
                      ^     ~~~~~~~~~~~~~~~~~~~~
1 warning and 1 error generated.

Fixes: c2c3912410 "ac/debug: annotate IB dumps with the raw values"
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Signed-off-by: Rob Herring <robh@kernel.org>