Include platform.h unconditionally: fixes undefined mbedtls_setbuf

Now that mbedtls/platform.h is included unconditionally, there are no more
configurations where mbedtls_setbuf was accidentally left out of the manual
definitions when MBEDTLS_PLATFORM_C is disabled. Fixes #6118, #6196.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/ChangeLog.d/platform-setbuf.txt b/ChangeLog.d/platform-setbuf.txt
new file mode 100644
index 0000000..844f70c
--- /dev/null
+++ b/ChangeLog.d/platform-setbuf.txt
@@ -0,0 +1,3 @@
+Bugfix
+   * Provide the missing definition of mbedtls_setbuf() in some configurations
+     with MBEDTLS_PLATFORM_C disabled. Fixes #6118, #6196.