Add missing 'explicit's on single-argument constructors.

Change-Id: I1494df6e74ec16238971fb4346ba184eb61c37ab
diff --git a/src/space.h b/src/space.h
index e354aab..d312365 100644
--- a/src/space.h
+++ b/src/space.h
@@ -92,7 +92,7 @@
   // create a Space from an existing memory mapping, taking ownership of the address space.
   static Space* Create(MemMap* mem_map);
 
-  Space(const std::string& name)
+  explicit Space(const std::string& name)
       : name_(name), mspace_(NULL), maximum_size_(0), image_header_(NULL), base_(0), limit_(0) {
   }