graphics/igt-gpu-tools: fix build on big-endian

../lib/igt_fb.c:4476:21: error: implicit declaration of function 'bswap_32' is invalid in C99 [-Werror,-Wimplicit-function-declaration]

uint32_t pixel = le32_to_cpu(line[x]);
                 ^

../lib/igt_core.h:1459:25: note: expanded from macro 'le32_to_cpu'