capsule-malloc: Remove duplicate typedef for mstate

I occasionally compile libcapsule with clang for better diagnostics.
Redefining typedefs is a C11 feature, which clang 8 won't allow in
C99 mode.

Signed-off-by: Simon McVittie <smcv@collabora.com>
3 jobs for master in 24 minutes and 31 seconds (queued for 12 minutes and 6 seconds)
Status Job ID Name Coverage
  Test
passed #15966
build:bionic

00:22:42

passed #15965
build:buster

00:22:19

passed #15964
build:stretch

00:23:44