BUILD: dumpstats: fix build error on Solaris

BUILD: dumpstats: fix build error on Solaris

Since the recent addition of map updates, haproxy does not build anymore
on Solaris because "s_addr" is a #define :

src/dumpstats.c: In function `stats_map_lookup':
src/dumpstats.c:4688: error: syntax error before '.' token
src/dumpstats.c:4781: error: `S_un' undeclared (first use in this function)
src/dumpstats.c:4781: error: (Each undeclared identifier is reported only once
src/dumpstats.c:4781: error: for each function it appears in.)
make: *** [src/dumpstats.o] Error 1

Simply rename the variable.