[winlocks] Include stdlib.h to avoid crash in Windows

With the missing include, VC makes assumptions about malloc(), free(), etc. in winlock's version of ssh_mutex_lock() which leads to a crash in InitializeCriticalSection().

Updates to the tests and documentation are not applicable with this small patch.

Checklist

• Commits have Signed-off-by: with name/author being identical to the commit author
• Code modified for feature
• Test suite updated with functionality tests
• Test suite updated with negative tests
• Documentation updated

Reviewer's checklist:

• Any issues marked for closing are addressed
• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent and according to CONTRIBUTING.md
• This feature/change has adequate documentation added
• No obvious mistakes in the code