util/printf: Expose util_printf_prev_tok

Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25753>

src/util/u_printf.h

@@ -37,6 +37,8 @@ typedef struct u_printf_info {
    char *strings;
 } u_printf_info;
 
+const char *util_printf_prev_tok(const char *str);
+
 /* find next valid printf specifier in a C string wrapper */
 size_t util_printf_next_spec_pos(const char *str, size_t pos);