selftests/exec: Use %zu to format size_t

On 32-bit:

execveat.c: In function 'check_execveat_pathmax':
execveat.c:183: warning: format '%lu' expects type 'long unsigned int', but argument 3 has type 'size_t'
execveat.c:187: warning: format '%lu' expects type 'long unsigned int', but argument 2 has type 'size_t'

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>