net: lib: http_server: fix snprintk issue of size_t

The size_t type may vary from machines. Current snprintk code was
causing below build issue on arm64.

error: format '%x' expects argument of type 'unsigned int', but argument
4 has type 'size_t' {aka 'long unsigned int'} [-Werror=format=].

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>