btstack_base64_encoder.h using POSIX only variable type

[simo-scar-cob]

Describe the bug

While compiling port WinUSB on develop branch, compiler raises error for variable type: ssize_t ; used in btstack_base64_encoder.h file.
Which acutally I think is only for POSIX systems (UNIX, Linux, etc..).
On Windows the equivalent I think is SSIZE_T , defined in #include <basetsd.h>.
This fix works for me, but I think that a separation based on OS is needed.

To Reproduce

Steps to reproduce the behavior:

1. open develop branch on Visual Studio 2022 Community
2. run compilation for windows-winusb port

Expected behavior
Compile success

Environment: (please complete the following information):

• Current BTstack branch: develop
• Bluetooth Controller: CSR 8510

[mringwal]

Thanks for reporting. We'll discuss internally, which way to go.

[mringwal]

We've added a typedef of SSIZE_T in btstack_defines.h on develop branch.