crash-reporter: change kDumpFormat to make it compile with gcc 4.7

BUG=None
TEST=compile passed

Change-Id: I9de18a4c91048881717731a88b265a09bfb22060
Reviewed-on: https://gerrit.chromium.org/gerrit/21687
Reviewed-by: Michael Krebs <mkrebs@chromium.org>
Tested-by: Yunlian Jiang <yunlian@chromium.org>
Commit-Ready: Yunlian Jiang <yunlian@chromium.org>
diff --git a/crash_reporter/kernel_collector.cc b/crash_reporter/kernel_collector.cc
index fc8500e..83eca8a 100644
--- a/crash_reporter/kernel_collector.cc
+++ b/crash_reporter/kernel_collector.cc
@@ -12,7 +12,7 @@
 static const char kDefaultKernelStackSignature[] =
     "kernel-UnspecifiedStackSignature";
 static const char kDumpPath[] = "/dev/pstore";
-static const char kDumpFormat[] = "dmesg-ramoops-%d";
+static const char kDumpFormat[] = "dmesg-ramoops-%zu";
 static const char kKernelExecName[] = "kernel";
 // Maximum number of records to examine in the kDumpPath.
 static const size_t kMaxDumpRecords = 100;