BUG/MINOR: change header-declared function to static inline

When we include the header proto/spoe.h in other files in the same
project, the compilator claim that the symbol have multiple definitions:

   src/flt_spoe.o: In function `spoe_encode_varint':
   ~/git/haproxy/include/proto/spoe.h:45: multiple definition of `spoe_encode_varint'
   src/proto_http.o:~/git/haproxy/include/proto/spoe.h:45: first defined here
1 file changed