Fix changelog requirements section.

Setbuf() is currently not guarded by MBEDTLS_FS_IO, nor can it easily be
so. Raised a seperate issue to cover this, and removed the mention of
MBEDTLS_FS_IO from the Changelog.

Signed-off-by: Paul Elliott <paul.elliott@arm.com>
diff --git a/ChangeLog.d/add_mbedtls_setbuf.txt b/ChangeLog.d/add_mbedtls_setbuf.txt
index 6152d60..d14cd18 100644
--- a/ChangeLog.d/add_mbedtls_setbuf.txt
+++ b/ChangeLog.d/add_mbedtls_setbuf.txt
@@ -5,6 +5,7 @@
      Glenn Strauss.
 Requirement changes
    * The library will no longer compile out of the box on a platform without
-     setbuf() if MBEDTLS_FS_IO is enabled. If your platform does not have
-     setbuf(), you can configure an alternative function by enabling
-     MBEDTLS_PLATFORM_SETBUF_ALT or MBEDTLS_PLATFORM_SETBUF_MACRO.
+     setbuf(). If your platform does not have setbuf(), you can configure an
+     alternative function by enabling MBEDTLS_PLATFORM_SETBUF_ALT or
+     MBEDTLS_PLATFORM_SETBUF_MACRO.
+