*: don't include standard C headers inside of extern "C"

Including C headers inside of `extern "C"` breaks use from C++. Hoist
the includes of standard C headers above the block so we don't try
to mangle the stdlib.

I initially tried to scope this with a targeted change but it's too
hard to do correctly that way. This way, we avoid whack-a-mole.

Firefox is working around this in their e21461b7b8b39cc31ba53c47d4f6f310c673ff2f
commit.

Bug: https://bugzilla.mozilla.org/1953080

spa/plugins/jack/jack-client.h

@@ -5,16 +5,16 @@
 #ifndef SPA_JACK_CLIENT_H
 #define SPA_JACK_CLIENT_H
 
-#ifdef __cplusplus
-extern "C" {
-#endif
-
 #include <spa/utils/defs.h>
 #include <spa/utils/hook.h>
 #include <spa/support/log.h>
 
 #include <jack/jack.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 struct spa_jack_client_events {
 #define SPA_VERSION_JACK_CLIENT_EVENTS	0
 	uint32_t version;