Fix for SVN 1.7 with sqlite >= 3.12.0 and VS 2005+

Hi,

this is merely for the record, in case someone else runs into the issue
and is looking here for a solution.

Note that SVN 1.7 is EOL and therefore isn't supported anymore. SVN
versions >= 1.8 don't suffer the problem.

The issue only applies under the following conditions:

- SVN 1.7
- SQLite 3.12.0+ (amalgamation)
- Visual Studio 2005+

In this case compiling libsvn_subr will trigger the following error:
[...]sqlite\sqlite3.c(42437): error C4013: 'rand_s' undefined; assuming
extern returning int
[[...]\1.7.22\build\win32\vcnet-vcproj\libsvn_subr.vcxproj]

The attached patch resolves the compile error (patch was suggested by
rhuijben).

Full details: http://www.luke1410.de:8090/browse/MAXSVN-55

Regards,
Stefan