selftests/nolibc: fix printf format mismatch in expect_str_buf_eq()

selftests/nolibc: fix printf format mismatch in expect_str_buf_eq()

Fix the following compiler warning on 32bit:

  i386-linux-gcc -Os -fno-ident -fno-asynchronous-unwind-tables -std=c89 -W -Wall -Wextra -fno-stack-protector -m32 -mstack-protector-guard=global -fstack-protector-all  -o nolibc-test \
    -nostdlib -nostdinc -static -Isysroot/i386/include nolibc-test.c nolibc-test-linkage.c -lgcc
  nolibc-test.c: In function 'expect_str_buf_eq':
  nolibc-test.c:610:30: error: format '%lu' expects argument of type 'long unsigned int', but argument 2 has type 'size_t' {aka 'unsigned int'} [-Werror=format=]
    610 |         llen += printf(" = %lu <%s> ", expr, buf);
        |                            ~~^         ~~~~
        |                              |         |
        |                              |         size_t {aka unsigned int}
        |                              long unsigned int
        |                            %u

Fixes: 1063649cf531 ("selftests/nolibc: Add tests for strlcat() and strlcpy()")
Cc: stable@vger.kernel.org
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>