slirp: fix unused return value, spotted by clang

Fix clang errors like:
  CC    slirp/cksum.o
/src/qemu/slirp/cksum.c:78:3: error: expression result unused [-Wunused-value]
                REDUCE;
/src/qemu/slirp/cksum.c:45:66: note: instantiated from:
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>

slirp/cksum.c

@@ -42,7 +42,8 @@
  */
 
 #define ADDCARRY(x)  (x > 65535 ? x -= 65535 : x)
-#define REDUCE {l_util.l = sum; sum = l_util.s[0] + l_util.s[1]; ADDCARRY(sum);}
+#define REDUCE {l_util.l = sum; sum = l_util.s[0] + l_util.s[1];        \
+        (void)ADDCARRY(sum);}
 
 int cksum(struct mbuf *m, int len)
 {