Fix presubmit style warning for gdb-jit.cc

src/gdb-jit.cc

Index: src/gdb-jit.cc
diff --git a/src/gdb-jit.cc b/src/gdb-jit.cc
index cf8ca2d4d9c8bceb1d7d14d61a2b7d51931159a9..bd663fb67494b325c2f925fcd1e26083bd77a5cb 100644
--- a/src/gdb-jit.cc
+++ b/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);
   }