util: Use unique type for QemuRecMutex in thread-posix.h

We will shortly convert lockable.h to _Generic, and we cannot
have two compatible types in the same expansion.  Wrap QemuMutex
in a struct, and unwrap in qemu-thread-posix.c.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20210614233143.1221879-6-richard.henderson@linaro.org>

include/qemu/thread-posix.h

@@ -4,8 +4,6 @@
 #include <pthread.h>
 #include <semaphore.h>
 
-typedef QemuMutex QemuRecMutex;
-
 struct QemuMutex {
     pthread_mutex_t lock;
 #ifdef CONFIG_DEBUG_MUTEX
@@ -15,6 +13,14 @@ struct QemuMutex {
     bool initialized;
 };
 
+/*
+ * QemuRecMutex cannot be a typedef of QemuMutex lest we have two
+ * compatible cases in _Generic.  See qemu/lockable.h.
+ */
+typedef struct QemuRecMutex {
+    QemuMutex m;
+} QemuRecMutex;
+
 struct QemuCond {
     pthread_cond_t cond;
     bool initialized;