Add conversion constructor for hidl_string.

Test: hidl_test
Change-Id: I61aaba51273ae9c55f77f1a2525df541a95c35fa
diff --git a/include/hidl/HidlSupport.h b/include/hidl/HidlSupport.h
index ec9123a..c3ec8f5 100644
--- a/include/hidl/HidlSupport.h
+++ b/include/hidl/HidlSupport.h
@@ -35,13 +35,15 @@
     ~hidl_string();
 
     hidl_string(const hidl_string &);
-    hidl_string &operator=(const hidl_string &);
+    hidl_string(const char *);
 
     const char *c_str() const;
     size_t size() const;
     bool empty() const;
 
+    hidl_string &operator=(const hidl_string &);
     hidl_string &operator=(const char *s);
+
     void clear();
 
     // Reference an external char array. Ownership is _not_ transferred.