util: Add extern "C" guard in simple_mtx.h

This is a header that can be accessed both in C/C++,
And when remove the usage of _MTX_INITIALIZER_NP in latter commits, it's need
implement simple_mtx_init in .c file, so for ABI consistence
Add the extern "C" for cpp files

Also add comment for #endif guard for code readability

Signed-off-by: Yonggang Luo <luoyonggang@gmail.com>
Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Reviewed-by: Jason Ekstrand <jason.ekstrand@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17122>

src/util/simple_mtx.h

@@ -40,6 +40,14 @@
 #  define HG(x)
 #endif
 
+#endif /* UTIL_FUTEX_SUPPORTED */
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#if UTIL_FUTEX_SUPPORTED
+
 /* mtx_t - Fast, simple mutex
  *
  * While modern pthread mutexes are very fast (implemented using futex), they
@@ -135,7 +143,7 @@ simple_mtx_assert_locked(simple_mtx_t *mtx)
    assert(mtx->val);
 }
 
-#else
+#else /* !UTIL_FUTEX_SUPPORTED */
 
 typedef mtx_t simple_mtx_t;
 
@@ -181,6 +189,10 @@ simple_mtx_assert_locked(simple_mtx_t *mtx)
 #endif
 }
 
+#endif /* UTIL_FUTEX_SUPPORTED */
+
+#ifdef __cplusplus
+}
 #endif
 
-#endif
+#endif /* _SIMPLE_MTX_H */