Fix MSVC version guard for C99 format size specifiers

Visual Studio 2013 (_MSC_VER == 1800) doesn't support %zu - only use it
on 2015 and above (_MSC_VER >= 1900).

%ldd works on Visual Studio 2013, but this patch keeps the two macro
definitions together, for simplicity's sake.

Signed-off-by: Bence Szépkúti <bence.szepkuti@arm.com>
diff --git a/ChangeLog.d/fix-msvc-version-guard-format-zu.txt b/ChangeLog.d/fix-msvc-version-guard-format-zu.txt
new file mode 100644
index 0000000..637388e
--- /dev/null
+++ b/ChangeLog.d/fix-msvc-version-guard-format-zu.txt
@@ -0,0 +1,5 @@
+Bugfix
+   * Fix definition of MBEDTLS_PRINTF_SIZET to prevent runtime crashes that
+     occurred whenever SSL debugging was enabled on a copy of Mbed TLS built
+     with Visual Studio 2013.
+     Fixes #10017.