Fix constant conversion warning in igmp

[dim]

Summary

With clang 3.9.0, compiling sys/netinet/igmp.c results in the following
warning:

sys/netinet/igmp.c:546:21: error: implicit conversion from 'int' to 'char' changes value from 148 to -108 [-Werror,-Wconstant-conversion]
        p->ipopt_list[0] = IPOPT_RA;    /* Router Alert Option */
                         ~ ^~~~~~~~
sys/netinet/ip.h:153:19: note: expanded from macro 'IPOPT_RA'
#define IPOPT_RA                148             /* router alert */
                                ^~~

This is because ipopt_list is an array of char, so IPOPT_RA is wrapped
to a negative value. It would be nice to change ipopt_list to an array
of u_char, but it looks like that changes the ABI. So I added an
explicit cast to suppress the warning. The bits that end up in
ipopt_list[0] will be 0x94, as expected.

[imp]

I wonder how changing this from char to uint8_t would be an ABI change though...

[dim]

In D7777#161380, @imp wrote:

I wonder how changing this from char to uint8_t would be an ABI change though...

Because ipopt_list is in struct ipoption, which is a globally exported struct in sys/netinet/ip_var.h:

struct ipoption {
        struct  in_addr ipopt_dst;      /* first-hop dst if source routed */
        char    ipopt_list[MAX_IPOPTLEN];       /* options proper */
};

I'd be happy to change it to uint8_t or u_char, which doesn't change the size, but maybe some external applications depending on this signature might break?