gcc 17 usual whack a mole game

Hi,
  it is that time of the year again. :-)

Using the latest gcc (still in development) I get this error:

```
...  
lyx/src/support/../support/docstring.h:55:14: error: ‘basic_ostringstream’ in namespace ‘std’ does not name a template type  
55 | typedef std::basic_ostringstream<char_type, std::char_traits<char_type>, std::allocator<char_type> > odocstringstream;  
|              ^~~~~~~~~~~~~~~~~~~  
/home/jamatos/lyx/lyx/src/support/../support/docstring.h:37:1: note: ‘std::basic_ostringstream’ is defined in header ‘<sstream>’; this is probably fixable by adding ‘#include <sstream>’  
36 | #include <string>  
+++ |+#include <sstream>  
37 |

```

Adding the suggested `sstream` include allows the compilation to succeed.

I do not remember if we have specific rules for the includes, from the `std`.
I would add it before `string` to keep the list ordered alphabetically.

Notice that this is not even in Fedora devel (rawhide), it will land there in 6 months -- January, so there is no rush. 

Best regards,
```
-- 
José Abílio

```