*: 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

test/pwtest.h

@@ -7,10 +7,6 @@
 #ifndef PWTEST_H
 #define PWTEST_H
 
-#ifdef __cplusplus
-extern "C" {
-#endif
-
 #include <limits.h>
 #include <stddef.h>
 #include <stdbool.h>
@@ -20,6 +16,10 @@ extern "C" {
 #include <spa/utils/dict.h>
 #include "spa/support/plugin.h"
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /**
  * \defgroup pwtest Test Suite
  * \brief `pwtest` is a test runner framework for PipeWire.