Backslash in href.

When having a backslash in a href like:
```
<a href="C:\boost">`C:\boost`</a>
```
we get in LaTeX the error:
```
(aa_8h.tex
! Undefined control sequence.
<argument> C:\boost

l.6 \href{C:\boost}
                   {\texttt{ {\ttfamily C\+:\textbackslash{}boost}}}
```
this can be overcome by escaping the backslash.
Note this is just an example, for an URI actually forward slashes should be used. It is just about the backslash not generating an error.

1 files changed, 1 insertions, 0 deletions

diff --git a/src/util.cpp b/src/util.cpp
index 3e49384..8816de1 100644
--- a/src/util.cpp
+++ b/src/util.cpp
@@ -7164,6 +7164,7 @@ QCString latexFilterURL(const char *s)
     {
       case '#':  t << "\\#"; break;
       case '%':  t << "\\%"; break;
+      case '\\':  t << "\\\\"; break;
       default:
         t << c;
         break;