Fix presubmit style warning for gdb-jit.cc

For many moons now I've been seeing this warning when running make x64.release.check:

  /home/wingo/src/v8/src/gdb-jit.cc:632:
     Single-argument constructors should be marked explicit.
     [runtime/explicit] [5]

This patch fixes it.

BUG=

Review URL: https://codereview.chromium.org/13724006
Patch from Andy Wingo <wingo@igalia.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@14153 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/gdb-jit.cc

@@ -629,7 +629,7 @@ class MachO BASE_EMBEDDED {
 #if defined(__ELF)
 class ELF BASE_EMBEDDED {
  public:
-  ELF(Zone* zone) : sections_(6, zone) {
+  explicit ELF(Zone* zone) : sections_(6, zone) {
     sections_.Add(new(zone) ELFSection("", ELFSection::TYPE_NULL, 0), zone);
     sections_.Add(new(zone) ELFStringTable(".shstrtab"), zone);
   }