libgo: redirect grep output in mkrsysinfo.sh to /dev/null

I noticed a stray useless output line when building libgo.
    
    Reviewed-on: https://go-review.googlesource.com/32294

From-SVN: r241655

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-6d9929a1641b180e724c2fdcdd55f6a254f1dec0
+7fb11c908ddab4932cc416f16657cec3bc878a1a
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/mkrsysinfo.sh

@@ -78,7 +78,7 @@ if grep '^const _epoll_data_offset ' ${OUT} >/dev/null 2>&1; then
   fi
 fi
 # Make sure EPOLLET is positive.
-if grep '^const _EPOLLET = [0-9]' gen-sysinfo.go; then
+if grep '^const _EPOLLET = [0-9]' gen-sysinfo.go > /dev/null 2>&1; then
   echo "const _EPOLLETpos = _EPOLLET" >> ${OUT}
 else
   echo "const _EPOLLETpos = 0x80000000" >> ${OUT}