mrib.c: don't use cpu_to_be32 outside of function

[cotequeiroz]

cpu_to_be32 is not a constant, so it can't be used outside of a function.

Compilation fails on little-endian machines with:

[ 11%] Building C object CMakeFiles/omcproxy.dir/src/mrib.c.o
In file included from /var/lib/buildbot/slaves/slave-lede-builds4/mipsel_74kc/build/sdk/build_dir/target-mipsel_74kc_musl/omcproxy-2017-02-14-1fe6f48f/src/omcproxy.h:51:0,
                 from /var/lib/buildbot/slaves/slave-lede-builds4/mipsel_74kc/build/sdk/build_dir/target-mipsel_74kc_musl/omcproxy-2017-02-14-1fe6f48f/src/mrib.c:39:
/var/lib/buildbot/slaves/slave-lede-builds4/mipsel_74kc/build/sdk/build_dir/target-mipsel_74kc_musl/omcproxy-2017-02-14-1fe6f48f/src/mrib.c:57:34: error: braced-group within expression allowed only inside a function
 static uint32_t ipv4_rtr_alert = cpu_to_be32(0x94040000);
                                  ^

Linux kernel defines __constant_cpu_to_be32 somewhere, but including its headers was messy. It is used only once, so it is simpler to just spell out the constant.

Signed-off-by: Eneas U de Queiroz cote2004-github@yahoo.com