reactor: Add missing CentOS 7.2 dependency systemtap-sdt-devel

This patch added systemtap-sdt-devel package to install-dependencies.sh
for centos.

Error message when building on centos7.2:
  [2/169] CXX build/release/core/reactor.o
  core/reactor.cc:85:21: fatal error: sys/sdt.h: No such file or directory

Env:
  # cat /etc/redhat-release
  CentOS Linux release 7.2.1511 (Core)

Signed-off-by: Javeme Lee <javaloveme@gmail.com>
Message-Id: <20170104145213.44721-1-javaloveme@gmail.com>

install-dependencies.sh

@@ -40,9 +40,9 @@ elif [ "$ID" = "centos" ] || [ "$ID" = "fedora" ]; then
         yum install -y epel-release
         curl -o /etc/yum.repos.d/scylla-1.2.repo http://downloads.scylladb.com/rpm/centos/scylla-1.2.repo
     fi
-    yum install -y libaio-devel hwloc-devel numactl-devel libpciaccess-devel cryptopp-devel libxml2-devel xfsprogs-devel gnutls-devel lksctp-tools-devel lz4-devel gcc make protobuf-devel protobuf-compiler libunwind-devel
+    yum install -y libaio-devel hwloc-devel numactl-devel libpciaccess-devel cryptopp-devel libxml2-devel xfsprogs-devel gnutls-devel lksctp-tools-devel lz4-devel gcc make protobuf-devel protobuf-compiler libunwind-devel systemtap-sdt-devel
     if [ "$ID" = "fedora" ]; then
-        dnf install -y gcc-c++ ninja-build ragel boost-devel xen-devel libubsan libasan systemtap-sdt-devel
+        dnf install -y gcc-c++ ninja-build ragel boost-devel xen-devel libubsan libasan
     else # centos
         yum install -y scylla-binutils scylla-gcc-c++ scylla-ninja-build scylla-ragel scylla-boost-devel scylla-libubsan scylla-libasan scylla-libstdc++-static python34
         echo "g++-5 is installed for Seastar. To build Seastar with g++-5, specify '--compiler=/opt/scylladb/bin/g++ --static-stdc++' on configure.py"