Add override to LogStreamBuf::overflow()

Prevents warnings such as:
```
glog/logging.h:1122:20: error: ‘virtual
std::basic_streambuf<char>::int_type
google::base_logging::LogStreamBuf::overflow(std::basic_streambuf<char>::int_type)’
can be marked override [-Werror=suggest-override]
   virtual int_type overflow(int_type ch) {
                    ^~~~~~~~
```

Signed-off-by: Santiago Gil <santiago.gil@codethink.co.uk>

src/glog/logging.h.in

@@ -1128,7 +1128,7 @@ class GOOGLE_GLOG_DLL_DECL LogStreamBuf : public std::streambuf {
   }
 
   // This effectively ignores overflow.
-  virtual int_type overflow(int_type ch) {
+  int_type overflow(int_type ch) {
     return ch;
   }
 

src/windows/glog/logging.h

@@ -1129,7 +1129,7 @@ class GOOGLE_GLOG_DLL_DECL LogStreamBuf : public std::streambuf {
   }
 
   // This effectively ignores overflow.
-  virtual int_type overflow(int_type ch) {
+  int_type overflow(int_type ch) {
     return ch;
   }